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squashfs read and write alternative and efficiently compress unallocated spaces #86

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opened 2025-08-03 20:29:08 +02:00 by Benjamin_Loison · 1 comment
Benjamin_Loison commented 2025-08-03 20:29:08 +02:00
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DuckDuckGo search Linux sparsify disk and Linux replace unused space with zeros.

The Unix Stack Exchange answer 44236 helps.

sudo zerofree

usage: zerofree [-n] [-v] [-f fillval] filesystem

sudo zerofree /media/benjamin_loison/d0XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXc0/

zerofree: failed to open filesystem /media/benjamin_loison/d0XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXc0/

sudo zerofree /media/benjamin_loison/disk/SSDServer.iso

zerofree: failed to open filesystem /media/benjamin_loison/disk/SSDServer.iso

file SSDServer-squash.img 

SSDServer-squash.img: Squashfs filesystem, little endian, version 4.0, zlib compressed, 89841958468 bytes, 2 inodes, blocksize: 131072 bytes, created: Sun Sep 11 23:26:39 2022

DuckDuckGo and Google search zerofill squashfs and "zerofill" "squashfs".

Squashfs is a compressed read-only file system for Linux.

Source: Wikipedia: SquashFS (1287034981)

Wikipedia: Comparison_of_file_systems#Block capabilities (1303557071) Deduplication and Compression)

Wikipedia: Comparison of file systems#Allocation and layout policies (1303557071) Sparse files may be relevant

DuckDuckGo and Google search Linux compressed read-write file system, Linux compress iso and dd without zeros.

dd --help
Output: 
Each CONV symbol may be:

...
  sparse    try to seek rather than write all-NUL output blocks

man dd
Output: 
       Each CONV symbol may be:

       ...

       sparse try to seek rather than write all-NUL output blocks

DuckDuckGo search "dd" "sparse".

FILE=/media/benjamin_loison/disk/SSDServer.iso; dd if=$FILE | pv --size `wc -c $FILE | cut -d ' ' -f 1` | sudo dd of=/media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso
Output: 
488280064+0 records iniB/s] [==================================================================================================================================================================>  ] 99% ETA 0:00:00
488280064+0 records out
249999392768 bytes (250 GB, 233 GiB) copied, 2467.93 s, 101 MB/s
 232GiB 0:41:07 [96.6MiB/s] [===================================================================================================================================================================>] 100%            
488280064+0 records in
488280064+0 records out
249999392768 bytes (250 GB, 233 GiB) copied, 2467.98 s, 101 MB/s

ls -lh /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso

-rw-r--r-- 1 root root 233G Aug  3 01:45 /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso

ls -l /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso

-rw-r--r-- 1 root root 249999392768 Aug  3 01:45 /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso

sudo zerofree --help
Output: 
zerofree: invalid option -- '-'
usage: zerofree [-n] [-v] [-f fillval] filesystem

man zerofree
Output: 
ZEROFREE(8)                                                                                System Manager's Manual                                                                                ZEROFREE(8)

NAME
       zerofree — zero free blocks from ext2, ext3 and ext4 file-systems

SYNOPSIS
       zerofree [-n]  [-v]  [-f fillval]  filesystem

DESCRIPTION
       zerofree finds the unallocated, blocks with non-zero value content in an ext2, ext3 or ext4 filesystem (e.g. /dev/hda1) and fills them with zeroes (or another octet of your choice).

       Filling  unused areas with zeroes is useful if the device on which this file-system resides is a disk image. In this case, depending on the type of disk image, a secondary utility may be able to re‐
       duce the size of the disk image after zerofree has been run.

       Filling unused areas may also be useful with solid-state drives (SSDs). On some SSDs, filling blocks with ones (0xFF) is reported to trigger Flash block erasure by the firmware,  possibly  giving  a
       write performance increase.

       The  usual  way  to  achieve  the same result (zeroing the unallocated blocks) is to run dd (1) to create a file full of zeroes that takes up the entire free space on the drive, and then delete this
       file. This has many disadvantages, which zerofree alleviates:

          •  it is slow;

          •  it makes the disk image (temporarily) grow to its maximal extent;

          •  it (temporarily) uses all free space on the disk, so other concurrent write actions may fail.

       filesystem has to be unmounted or mounted read-only for zerofree to work. It will exit with an error message if the filesystem is mounted writable. To remount the root file-system readonly, you  can
       first switch to single user runlevel (telinit 1) then use mount -o remount,ro filesystem.

       zerofree has been written to be run from GNU/Linux systems installed as guest OSes inside a virtual machine. In this case, it is typically run from within the guest system, and a utility is then run
       from the host system to shrink disk image (VBoxManage modifyhd --compact, provided with virtualbox, is able to do that for some disk image formats).

       It may however be useful in other situations: for instance it can be used to make it more difficult to retrieve deleted data. Beware that securely deleting sensitive data is not in general  an  easy
       task and usually requires writing several times on the deleted blocks.

OPTIONS
       -n        Perform a dry run  (do not modify the file-system);

       -v        Be verbose: show the number of blocks modified by zerofree (or that would be modified, in case the -n is used), the number of free blocks and the total number of blocks on the filesystem;

       -f value  Specify the octet value to fill empty blocks with (defaults to 0). Argument must be within the range 0 to 255.

SEE ALSO
       dd (1).

AUTHOR
       This manual page was written by Thibaut Paumard <paumard@users.sourceforge.net> for the Debian system (but may be used by others).  Permission is granted to copy, distribute and/or modify this docu‐
       ment under the terms of the GNU General Public License, Version 2 or any later version published by the Free Software Foundation.

       On Debian systems, the complete text of the GNU General Public License can be found in /usr/share/common-licenses/GPL-2.

                                                                                                                                                                                                  ZEROFREE(8)

time sudo zerofree -v /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso
Output: 
14709203/16204235/61035008

real	4m24.833s
user	0m0.026s
sys	0m0.058s

DuckDuckGo search zerofree output meaning.

Based on man 14709203 / 61035008 = 24 % are now zero, so can earn at most 256 * 24 / 100 = 61.4 GB.

Unclear if modified means where not value to write.

mksquashfs
Output: 
SYNTAX:mksquashfs source1 source2 ...  FILESYSTEM [OPTIONS] [-e list of
exclude dirs/files]

Filesystem build options:
-tar			read uncompressed tar file from standard in (stdin)
-no-strip		act like tar, and do not strip leading directories
			from source files
-tarstyle		alternative name for -no-strip
-cpiostyle		act like cpio, and read files from standard in (stdin)
-cpiostyle0		like -cpiostyle, but filenames are null terminated
-comp <comp>		select <comp> compression
			Compressors available:
				gzip (default)
				lzo
				lz4
				xz
				zstd
				lzma
-b <block_size>		set data block to <block_size>.  Default 128 Kbytes.
			Optionally a suffix of K or M can be given to specify
			Kbytes or Mbytes respectively
-reproducible		build filesystems that are reproducible (default)
-not-reproducible	build filesystems that are not reproducible
-mkfs-time <time>	set filesystem creation timestamp to <time>, which is
			an unsigned 32-bit int indicating seconds since the
			epoch (1970-01-01)
-fstime <time>		synonym for mkfs-time
-all-time <time>	set all file timestamps to <time>, which is an unsigned
			32-bit int indicating seconds since the epoch
			(1970-01-01)
-no-exports		don't make filesystem exportable via NFS (-tar default)
-exports		make filesystem exportable via NFS (default)
-no-sparse		don't detect sparse files
-no-xattrs		don't store extended attributes
-xattrs			store extended attributes (default)
-noI			do not compress inode table
-noId			do not compress the uid/gid table (implied by -noI)
-noD			do not compress data blocks
-noF			do not compress fragment blocks
-noX			do not compress extended attributes
-no-tailends		don't pack tail ends into fragments (default)
-tailends		pack tail ends into fragments
-no-fragments		do not use fragments
-always-use-fragments	use fragment blocks for files larger than block size
-no-duplicates		do not perform duplicate checking
-no-hardlinks		do not hardlink files, instead store duplicates
-all-root		make all files owned by root
-root-time <time>	set root directory time to <time>
-root-mode <mode>	set root directory permissions to octal <mode>
-root-uid <uid>		set root directory owner to <uid>
-root-gid <gid>		set root directory group to <gid>
-force-uid <uid>	set all file uids to <uid>
-force-gid <gid>	set all file gids to <gid>
-keep-as-directory	if one source directory is specified, create a root
			directory containing that directory, rather than the
			contents of the directory
-action <action@expr>	evaluate <expr> on every file, and execute <action>
			if it returns TRUE
-log-action <act@expr>	as above, but log expression evaluation results and
			actions performed
-true-action <act@expr>	as above, but only log expressions which return TRUE
-false-action <act@exp>	as above, but only log expressions which return FALSE
-action-file <file>	as action, but read actions from <file>
-log-action-file <file>	as -log-action, but read actions from <file>
-true-action-file <f>	as -true-action, but read actions from <f>
-false-action-file <f>	as -false-action, but read actions from <f>

Filesystem filter options:
-p <pseudo-definition>	Add pseudo file definition.  The definition should
			be quoted
-pf <pseudo-file>	Add list of pseudo file definitions.  Pseudo file
			definitions in pseudo-files should not be quoted
-sort <sort_file>	sort files according to priorities in <sort_file>.  One
			file or dir with priority per line.  Priority -32768 to
			32767, default priority 0
-ef <exclude_file>	list of exclude dirs/files.  One per line
-wildcards		Allow extended shell wildcards (globbing) to be used in
			exclude dirs/files
-regex			Allow POSIX regular expressions to be used in exclude
			dirs/files
-one-file-system	Do not cross filesystem boundaries when scanning sources

Filesystem append options:
-noappend		do not append to existing filesystem
-root-becomes <name>	when appending source files/directories, make the
			original root become a subdirectory in the new root
			called <name>, rather than adding the new source items
			to the original root

Mksquashfs runtime options:
-version		print version, licence and copyright message
-exit-on-error		treat normally ignored errors as fatal
-recover <name>		recover filesystem data using recovery file <name>
-no-recovery		don't generate a recovery file
-recovery-path <name>	use <name> as the directory to store the recovery file
-quiet			no verbose output
-info			print files written to filesystem
-no-progress		don't display the progress bar
-progress		display progress bar when using the -info option
-throttle <percentage>	throttle the I/O input rate by the given percentage.
			This can be used to reduce the I/O and CPU consumption
			of Mksquashfs
-limit <percentage>	limit the I/O input rate to the given percentage.
			This can be used to reduce the I/O and CPU consumption
			of Mksquashfs (alternative to -throttle)
-processors <number>	Use <number> processors.  By default will use number of
			processors available
-mem <size>		Use <size> physical memory.  Currently set to 3917M
			Optionally a suffix of K, M or G can be given to specify
			Kbytes, Mbytes or Gbytes respectively

Expert options (these may make the filesystem unmountable):
-nopad			do not pad filesystem to a multiple of 4K
-offset <offset>	Skip <offset> bytes at the beginning of FILESYSTEM.
			Optionally a suffix of K, M or G can be given to specify
			Kbytes, Mbytes or Gbytes respectively.
			Default 0 bytes
-o <offset>		synonym for -offset

Miscellaneous options:
-root-owned		alternative name for -all-root
-noInodeCompression	alternative name for -noI
-noIdTableCompression	alternative name for -noId
-noDataCompression	alternative name for -noD
-noFragmentCompression	alternative name for -noF
-noXattrCompression	alternative name for -noX

-help			output this options text to stdout
-h			output this options text to stdout

-Xhelp			print compressor options for selected compressor

Pseudo file definition format:
"filename d mode uid gid"		create a directory
"filename m mode uid gid"		modify filename
"filename b mode uid gid major minor"	create a block device
"filename c mode uid gid major minor"	create a character device
"filename f mode uid gid command"	create file from stdout of command
"filename s mode uid gid symlink"	create a symbolic link
"filename i mode uid gid [s|f]"		create a socket (s) or FIFO (f)
"filename l linkname"			create a hard-link to linkname
"filename L pseudo_filename"		same, but link to pseudo file
"filename D time mode uid gid"		create a directory with timestamp time
"filename M time mode uid gid"		modify a file with timestamp time
"filename B time mode uid gid major minor"
					create block device with timestamp time
"filename C time mode uid gid major minor"
					create char device with timestamp time
"filename F time mode uid gid command"	create file with timestamp time
"filename S time mode uid gid symlink"	create symlink with timestamp time
"filename I time mode uid gid [s|f]"	create socket/fifo with timestamp time

Compressors available and compressor specific options:
	gzip (default)
	  -Xcompression-level <compression-level>
		<compression-level> should be 1 .. 9 (default 9)
	  -Xwindow-size <window-size>
		<window-size> should be 8 .. 15 (default 15)
	  -Xstrategy strategy1,strategy2,...,strategyN
		Compress using strategy1,strategy2,...,strategyN in turn
		and choose the best compression.
		Available strategies: default, filtered, huffman_only,
		run_length_encoded and fixed
	lzo
	  -Xalgorithm <algorithm>
		Where <algorithm> is one of:
			lzo1x_1
			lzo1x_1_11
			lzo1x_1_12
			lzo1x_1_15
			lzo1x_999 (default)
	  -Xcompression-level <compression-level>
		<compression-level> should be 1 .. 9 (default 8)
		Only applies to lzo1x_999 algorithm
	lz4
	  -Xhc
		Compress using LZ4 High Compression
	xz
	  -Xbcj filter1,filter2,...,filterN
		Compress using filter1,filter2,...,filterN in turn
		(in addition to no filter), and choose the best compression.
		Available filters: x86, arm, armthumb, powerpc, sparc, ia64
	  -Xdict-size <dict-size>
		Use <dict-size> as the XZ dictionary size.  The dictionary size
		can be specified as a percentage of the block size, or as an
		absolute value.  The dictionary size must be less than or equal
		to the block size and 8192 bytes or larger.  It must also be
		storable in the xz header as either 2^n or as 2^n+2^(n+1).
		Example dict-sizes are 75%, 50%, 37.5%, 25%, or 32K, 16K, 8K
		etc.
	zstd
	  -Xcompression-level <compression-level>
		<compression-level> should be 1 .. 22 (default 15)
	lzma
	  (no options) (deprecated - no kernel support)

Environment:
SOURCE_DATE_EPOCH	If set, this is used as the filesystem creation
			timestamp.  Also any file timestamps which are
			after SOURCE_DATE_EPOCH will be clamped to
			SOURCE_DATE_EPOCH.  See
			https://reproducible-builds.org/docs/source-date-epoch/
			for more information

See also:
The README for the Squash-tools 4.5.1 release, describing the new features can be
read here https://github.com/plougher/squashfs-tools/blob/master/README-4.5.1

The Squashfs-tools USAGE guide can be read here
https://github.com/plougher/squashfs-tools/blob/master/USAGE

The ACTIONS-README file describing how to use the new actions feature can be
read here https://github.com/plougher/squashfs-tools/blob/master/ACTIONS-README

ls -lh /media/benjamin_loison/64XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXd8/SSDServer-squash.img

-rw-r--r-- 1 benjamin_loison benjamin_loison 84G Sep 12  2022 /media/benjamin_loison/64XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXd8/SSDServer-squash.img

ls -l /media/benjamin_loison/64XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXd8/SSDServer-squash.img

-rw-r--r-- 1 benjamin_loison benjamin_loison 89841958912 Sep 12  2022 /media/benjamin_loison/64XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXd8/SSDServer-squash.img

time sudo mksquashfs /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer{,_squash}.iso
Output: 
Parallel mksquashfs: Using 24 processors
Creating 4.0 filesystem on /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso, block size 131072.
[==========================================================================================================================================================================================|] 1907344/1907344 100%>

Exportable Squashfs 4.0 filesystem, gzip compressed, data block size 131072
	compressed data, compressed metadata, compressed fragments,
	compressed xattrs, compressed ids
	duplicates are removed
Filesystem size 67064143.13 Kbytes (65492.33 Mbytes)
	27.47% of uncompressed filesystem size (244147484.63 Kbytes)
Inode table size 2776566 bytes (2711.49 Kbytes)
	36.38% of uncompressed inode table size (7631328 bytes)
Directory table size 33 bytes (0.03 Kbytes)
	94.29% of uncompressed directory table size (35 bytes)
Number of duplicate files found 0
Number of inodes 2
Number of files 1
Number of fragments 0
Number of symbolic links 0
Number of device nodes 0
Number of fifo nodes 0
Number of socket nodes 0
Number of directories 1
Number of hard-links 0
Number of ids (unique uids + gids) 1
Number of uids 1
	root (0)
Number of gids 1
	root (0)

real	25m48.123s
user	0m0.107s
sys	0m0.170s

ls -lh /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso

-rw-r--r-- 1 root root 64G Aug  3 02:25 /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso

ls -l /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso

-rw-r--r-- 1 root root 68673683456 Aug  3 02:25 /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso

so it spared 20 GB!

DuckDuckGo search squashfs writable.

The Unix Stack Exchange answer 80312 is inappropriate in my case as recrompressing is an expensive operation.

Related to Benjamin_Loison/virt-manager/issues/31.

DuckDuckGo search btrfs compress iso, compress iso while keeping writable and squashfs writable alternative.

https://bbs.archlinux.org/viewtopic.php?id=131678

DuckDuckGo and Google search btrfs compress file and Use btrfs like squashfs.

sudo mkfs.ext4 test.iso
Output: 
mke2fs 1.47.0 (5-Feb-2023)
The file test.iso does not exist and no size was specified.

DuckDuckGo and Google search Linux create ext4 as file.

The Unix Stack Exchange question 753111

df -h /
Output: 
Filesystem                    Size  Used Avail Use% Mounted on
/dev/mapper/pegasus--vg-root  731G  554G  141G  80% /

fallocate -l 5G test.iso
sudo mkfs.ext4 test.iso
Output: 
mke2fs 1.47.0 (5-Feb-2023)
Discarding device blocks: done                            
Creating filesystem with 1310720 4k blocks and 327680 inodes
Filesystem UUID: 515a90d2-ca70-4939-9292-8f9c4c6fee6e
Superblock backups stored on blocks: 
	32768, 98304, 163840, 229376, 294912, 819200, 884736

Allocating group tables: done                            
Writing inode tables: done                            
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

df -h /
Output: 
Filesystem                    Size  Used Avail Use% Mounted on
/dev/mapper/pegasus--vg-root  731G  554G  141G  80% /

ls -lh test.iso 

-rw-r--r-- 1 benjamin_loison benjamin_loison 5.0G Aug  3 02:45 test.iso

sudo mkfs.btrfs
Output: 
btrfs-progs v6.2
See http://btrfs.wiki.kernel.org for more information.

usage: mkfs.btrfs [options] <dev> [<dev...>]

    Create a BTRFS filesystem on a device or multiple devices

    Allocation profiles:
    -d|--data PROFILE         data profile, raid0, raid1, raid1c3, raid1c4, raid5, raid6, raid10, dup or single
    -m|--metadata PROFILE     metadata profile, values like for data profile 
    -M|--mixed                mix metadata and data together 
    Features:
    --csum TYPE               
    --checksum TYPE           checksum algorithm to use, crc32c (default), xxhash, sha256, blake2 
    -n|--nodesize SIZE        size of btree nodes 
    -s|--sectorsize SIZE      data block size (may not be mountable by current kernel) 
    -O|--features LIST        comma separated list of filesystem features (use '-O list-all' to list features) 
    -R|--runtime-features LIST
                              comma separated list of runtime features (use '-R list-all' to list runtime features)
    -L|--label LABEL          set the filesystem label 
    -U|--uuid UUID            specify the filesystem UUID (must be unique) 
    Creation:
    -b|--byte-count SIZE      set size of each device to SIZE (filesystem size is sum of all device sizes) 
    -r|--rootdir DIR          copy files from DIR to the image root directory 
    --shrink                  (with --rootdir) shrink the filled filesystem to minimal size 
    -K|--nodiscard            do not perform whole device TRIM 
    -f|--force                force overwrite of existing filesystem 
    General:
    -q|--quiet                no messages except errors 
    -v|--verbose              increase verbosity level, default is 1 
    -V|--version              print the mkfs.btrfs version and exit 
    --help                    print this help and exit 
    Deprecated:
    -l|--leafsize SIZE        removed in 6.0, use --nodesize

file test.iso

test.iso: Linux rev 1.0 ext4 filesystem data, UUID=167c294b-86f3-41ef-9e16-f89a05e535e5 (extents) (64bit) (large files) (huge files)

mkdir a/
mount test.iso a/

mount: a/: failed to setup loop device for /home/benjamin_loison/test.iso.

sudo mount test.iso a/

does not return anything.

ls -lha a/
Output: 
total 24K
drwxr-xr-x   3 root            root            4.0K Aug  3 02:48 .
drwx--x---+ 44 benjamin_loison benjamin_loison 4.0K Aug  3 02:48 ..
drwx------   2 root            root             16K Aug  3 02:48 lost+found

umount a/

umount: /home/benjamin_loison/a: must be superuser to unmount.

sudo umount a/

does not return anything.

sudo mkfs.btrfs test.iso
Output: 
btrfs-progs v6.2
See http://btrfs.wiki.kernel.org for more information.

NOTE: several default settings have changed in version 5.15, please make sure
      this does not affect your deployments:
      - DUP for metadata (-m dup)
      - enabled no-holes (-O no-holes)
      - enabled free-space-tree (-R free-space-tree)

Label:              (null)
UUID:               b7095905-987f-444b-af34-573aa57d42cd
Node size:          16384
Sector size:        4096
Filesystem size:    5.00GiB
Block group profiles:
  Data:             single            8.00MiB
  Metadata:         DUP             256.00MiB
  System:           DUP               8.00MiB
SSD detected:       no
Zoned device:       no
Incompat features:  extref, skinny-metadata, no-holes
Runtime features:   free-space-tree
Checksum:           crc32c
Number of devices:  1
Devices:
   ID        SIZE  PATH
    1     5.00GiB  test.iso

sudo mount test.iso a/

does not return anything.

ls -lah a/
Output: 
total 20K
drwxr-xr-x   1 root            root               0 Aug  3 02:51 .
drwx--x---+ 44 benjamin_loison benjamin_loison 4.0K Aug  3 02:51 ..

DuckDuckGo and Google search btrfs enable compression.

https://btrfs.readthedocs.io/en/latest/Compression.html

I suspect that if the compression is at the file level, then modifying the big file, requires to recompress the file.

Typically the compression can be enabled on the whole filesystem, specified for the mount point.

The command above will start defragmentation of the whole file and apply the compression, regardless of the mount option.

With ext4:

sudo dd if=/dev/urandom of=a/my_data status=progress
Output: 
2017192960 bytes (2.0 GB, 1.9 GiB) copied, 16 s, 126 MB/s^C
4026336+0 records in
4026335+0 records out
2061483520 bytes (2.1 GB, 1.9 GiB) copied, 16.3456 s, 126 MB/s

df -h /
Output: 
Filesystem                    Size  Used Avail Use% Mounted on
/dev/mapper/pegasus--vg-root  731G  556G  139G  81% /

so I notice the 2 GB change.

The Super User answer 1136358 may help.

FILE=/media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso; dd if=$FILE | pv -s `wc -c $FILE | cut -d ' ' -f 1` | dd of=/media/benjamin_loison/64XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXd8/SSDServer-squash_zerofreed.img
Output: 
134128288+0 records iniB/s] [==================================================================================================================================================================>  ] 99% ETA 0:00:00
134128288+0 records out
68673683456 bytes (69 GB, 64 GiB) copied, 1344.43 s, 51.1 MB/s
64.0GiB 0:22:24 [48.7MiB/s] [===================================================================================================================================================================>] 100%            
134128288+0 records in
134128288+0 records out
68673683456 bytes (69 GB, 64 GiB) copied, 1343.55 s, 51.1 MB/s
DuckDuckGo search *Linux sparsify disk* and *Linux replace unused space with zeros*. [The Unix Stack Exchange answer 44236](https://unix.stackexchange.com/a/44236) helps. ``` sudo zerofree ``` ``` usage: zerofree [-n] [-v] [-f fillval] filesystem ``` ``` sudo zerofree /media/benjamin_loison/d0XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXc0/ ``` ``` zerofree: failed to open filesystem /media/benjamin_loison/d0XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXc0/ ``` ``` sudo zerofree /media/benjamin_loison/disk/SSDServer.iso ``` ``` zerofree: failed to open filesystem /media/benjamin_loison/disk/SSDServer.iso ``` ``` file SSDServer-squash.img ``` ``` SSDServer-squash.img: Squashfs filesystem, little endian, version 4.0, zlib compressed, 89841958468 bytes, 2 inodes, blocksize: 131072 bytes, created: Sun Sep 11 23:26:39 2022 ``` DuckDuckGo and Google search *zerofill squashfs* and `"zerofill" "squashfs"`. > Squashfs is a compressed read-only file system for Linux. Source: [Wikipedia: SquashFS (1287034981)](https://en.wikipedia.org/w/index.php?title=SquashFS&oldid=1287034981) [Wikipedia: Comparison_of_file_systems#Block capabilities (1303557071)](https://en.wikipedia.org/w/index.php?title=Comparison_of_file_systems&oldid=1303557071#Block_capabilities) *Deduplication* and *Compression*) [Wikipedia: Comparison of file systems#Allocation and layout policies (1303557071)](https://en.wikipedia.org/w/index.php?title=Comparison_of_file_systems&oldid=1303557071#Allocation_and_layout_policies) *Sparse files* may be relevant DuckDuckGo and Google search *Linux compressed read-write file system*, *Linux compress iso* and *dd without zeros*. ```bash dd --help ``` <details> <summary>Output:</summary> ``` Each CONV symbol may be: ... sparse try to seek rather than write all-NUL output blocks ``` </details> ``` man dd ``` <details> <summary>Output:</summary> ``` Each CONV symbol may be: ... sparse try to seek rather than write all-NUL output blocks ``` </details> DuckDuckGo search `"dd" "sparse"`. ```bash FILE=/media/benjamin_loison/disk/SSDServer.iso; dd if=$FILE | pv --size `wc -c $FILE | cut -d ' ' -f 1` | sudo dd of=/media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso ``` <details> <summary>Output:</summary> ``` 488280064+0 records iniB/s] [==================================================================================================================================================================> ] 99% ETA 0:00:00 488280064+0 records out 249999392768 bytes (250 GB, 233 GiB) copied, 2467.93 s, 101 MB/s 232GiB 0:41:07 [96.6MiB/s] [===================================================================================================================================================================>] 100% 488280064+0 records in 488280064+0 records out 249999392768 bytes (250 GB, 233 GiB) copied, 2467.98 s, 101 MB/s ``` </details> ``` ls -lh /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso ``` ``` -rw-r--r-- 1 root root 233G Aug 3 01:45 /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso ``` ``` ls -l /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso ``` ``` -rw-r--r-- 1 root root 249999392768 Aug 3 01:45 /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso ``` ``` sudo zerofree --help ``` <details> <summary>Output:</summary> ``` zerofree: invalid option -- '-' usage: zerofree [-n] [-v] [-f fillval] filesystem ``` </details> ``` man zerofree ``` <details> <summary>Output:</summary> ``` ZEROFREE(8) System Manager's Manual ZEROFREE(8) NAME zerofree — zero free blocks from ext2, ext3 and ext4 file-systems SYNOPSIS zerofree [-n] [-v] [-f fillval] filesystem DESCRIPTION zerofree finds the unallocated, blocks with non-zero value content in an ext2, ext3 or ext4 filesystem (e.g. /dev/hda1) and fills them with zeroes (or another octet of your choice). Filling unused areas with zeroes is useful if the device on which this file-system resides is a disk image. In this case, depending on the type of disk image, a secondary utility may be able to re‐ duce the size of the disk image after zerofree has been run. Filling unused areas may also be useful with solid-state drives (SSDs). On some SSDs, filling blocks with ones (0xFF) is reported to trigger Flash block erasure by the firmware, possibly giving a write performance increase. The usual way to achieve the same result (zeroing the unallocated blocks) is to run dd (1) to create a file full of zeroes that takes up the entire free space on the drive, and then delete this file. This has many disadvantages, which zerofree alleviates: • it is slow; • it makes the disk image (temporarily) grow to its maximal extent; • it (temporarily) uses all free space on the disk, so other concurrent write actions may fail. filesystem has to be unmounted or mounted read-only for zerofree to work. It will exit with an error message if the filesystem is mounted writable. To remount the root file-system readonly, you can first switch to single user runlevel (telinit 1) then use mount -o remount,ro filesystem. zerofree has been written to be run from GNU/Linux systems installed as guest OSes inside a virtual machine. In this case, it is typically run from within the guest system, and a utility is then run from the host system to shrink disk image (VBoxManage modifyhd --compact, provided with virtualbox, is able to do that for some disk image formats). It may however be useful in other situations: for instance it can be used to make it more difficult to retrieve deleted data. Beware that securely deleting sensitive data is not in general an easy task and usually requires writing several times on the deleted blocks. OPTIONS -n Perform a dry run (do not modify the file-system); -v Be verbose: show the number of blocks modified by zerofree (or that would be modified, in case the -n is used), the number of free blocks and the total number of blocks on the filesystem; -f value Specify the octet value to fill empty blocks with (defaults to 0). Argument must be within the range 0 to 255. SEE ALSO dd (1). AUTHOR This manual page was written by Thibaut Paumard <paumard@users.sourceforge.net> for the Debian system (but may be used by others). Permission is granted to copy, distribute and/or modify this docu‐ ment under the terms of the GNU General Public License, Version 2 or any later version published by the Free Software Foundation. On Debian systems, the complete text of the GNU General Public License can be found in /usr/share/common-licenses/GPL-2. ZEROFREE(8) ``` </details> ``` time sudo zerofree -v /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer.iso ``` <details> <summary>Output:</summary> ``` 14709203/16204235/61035008 real 4m24.833s user 0m0.026s sys 0m0.058s ``` </details> DuckDuckGo search *zerofree output meaning*. Based on `man` 14709203 / 61035008 = 24 % are now zero, so can earn at most 256 * 24 / 100 = 61.4 GB. Unclear if modified means where not value to write. ``` mksquashfs ``` <details> <summary>Output:</summary> ``` SYNTAX:mksquashfs source1 source2 ... FILESYSTEM [OPTIONS] [-e list of exclude dirs/files] Filesystem build options: -tar read uncompressed tar file from standard in (stdin) -no-strip act like tar, and do not strip leading directories from source files -tarstyle alternative name for -no-strip -cpiostyle act like cpio, and read files from standard in (stdin) -cpiostyle0 like -cpiostyle, but filenames are null terminated -comp <comp> select <comp> compression Compressors available: gzip (default) lzo lz4 xz zstd lzma -b <block_size> set data block to <block_size>. Default 128 Kbytes. Optionally a suffix of K or M can be given to specify Kbytes or Mbytes respectively -reproducible build filesystems that are reproducible (default) -not-reproducible build filesystems that are not reproducible -mkfs-time <time> set filesystem creation timestamp to <time>, which is an unsigned 32-bit int indicating seconds since the epoch (1970-01-01) -fstime <time> synonym for mkfs-time -all-time <time> set all file timestamps to <time>, which is an unsigned 32-bit int indicating seconds since the epoch (1970-01-01) -no-exports don't make filesystem exportable via NFS (-tar default) -exports make filesystem exportable via NFS (default) -no-sparse don't detect sparse files -no-xattrs don't store extended attributes -xattrs store extended attributes (default) -noI do not compress inode table -noId do not compress the uid/gid table (implied by -noI) -noD do not compress data blocks -noF do not compress fragment blocks -noX do not compress extended attributes -no-tailends don't pack tail ends into fragments (default) -tailends pack tail ends into fragments -no-fragments do not use fragments -always-use-fragments use fragment blocks for files larger than block size -no-duplicates do not perform duplicate checking -no-hardlinks do not hardlink files, instead store duplicates -all-root make all files owned by root -root-time <time> set root directory time to <time> -root-mode <mode> set root directory permissions to octal <mode> -root-uid <uid> set root directory owner to <uid> -root-gid <gid> set root directory group to <gid> -force-uid <uid> set all file uids to <uid> -force-gid <gid> set all file gids to <gid> -keep-as-directory if one source directory is specified, create a root directory containing that directory, rather than the contents of the directory -action <action@expr> evaluate <expr> on every file, and execute <action> if it returns TRUE -log-action <act@expr> as above, but log expression evaluation results and actions performed -true-action <act@expr> as above, but only log expressions which return TRUE -false-action <act@exp> as above, but only log expressions which return FALSE -action-file <file> as action, but read actions from <file> -log-action-file <file> as -log-action, but read actions from <file> -true-action-file <f> as -true-action, but read actions from <f> -false-action-file <f> as -false-action, but read actions from <f> Filesystem filter options: -p <pseudo-definition> Add pseudo file definition. The definition should be quoted -pf <pseudo-file> Add list of pseudo file definitions. Pseudo file definitions in pseudo-files should not be quoted -sort <sort_file> sort files according to priorities in <sort_file>. One file or dir with priority per line. Priority -32768 to 32767, default priority 0 -ef <exclude_file> list of exclude dirs/files. One per line -wildcards Allow extended shell wildcards (globbing) to be used in exclude dirs/files -regex Allow POSIX regular expressions to be used in exclude dirs/files -one-file-system Do not cross filesystem boundaries when scanning sources Filesystem append options: -noappend do not append to existing filesystem -root-becomes <name> when appending source files/directories, make the original root become a subdirectory in the new root called <name>, rather than adding the new source items to the original root Mksquashfs runtime options: -version print version, licence and copyright message -exit-on-error treat normally ignored errors as fatal -recover <name> recover filesystem data using recovery file <name> -no-recovery don't generate a recovery file -recovery-path <name> use <name> as the directory to store the recovery file -quiet no verbose output -info print files written to filesystem -no-progress don't display the progress bar -progress display progress bar when using the -info option -throttle <percentage> throttle the I/O input rate by the given percentage. This can be used to reduce the I/O and CPU consumption of Mksquashfs -limit <percentage> limit the I/O input rate to the given percentage. This can be used to reduce the I/O and CPU consumption of Mksquashfs (alternative to -throttle) -processors <number> Use <number> processors. By default will use number of processors available -mem <size> Use <size> physical memory. Currently set to 3917M Optionally a suffix of K, M or G can be given to specify Kbytes, Mbytes or Gbytes respectively Expert options (these may make the filesystem unmountable): -nopad do not pad filesystem to a multiple of 4K -offset <offset> Skip <offset> bytes at the beginning of FILESYSTEM. Optionally a suffix of K, M or G can be given to specify Kbytes, Mbytes or Gbytes respectively. Default 0 bytes -o <offset> synonym for -offset Miscellaneous options: -root-owned alternative name for -all-root -noInodeCompression alternative name for -noI -noIdTableCompression alternative name for -noId -noDataCompression alternative name for -noD -noFragmentCompression alternative name for -noF -noXattrCompression alternative name for -noX -help output this options text to stdout -h output this options text to stdout -Xhelp print compressor options for selected compressor Pseudo file definition format: "filename d mode uid gid" create a directory "filename m mode uid gid" modify filename "filename b mode uid gid major minor" create a block device "filename c mode uid gid major minor" create a character device "filename f mode uid gid command" create file from stdout of command "filename s mode uid gid symlink" create a symbolic link "filename i mode uid gid [s|f]" create a socket (s) or FIFO (f) "filename l linkname" create a hard-link to linkname "filename L pseudo_filename" same, but link to pseudo file "filename D time mode uid gid" create a directory with timestamp time "filename M time mode uid gid" modify a file with timestamp time "filename B time mode uid gid major minor" create block device with timestamp time "filename C time mode uid gid major minor" create char device with timestamp time "filename F time mode uid gid command" create file with timestamp time "filename S time mode uid gid symlink" create symlink with timestamp time "filename I time mode uid gid [s|f]" create socket/fifo with timestamp time Compressors available and compressor specific options: gzip (default) -Xcompression-level <compression-level> <compression-level> should be 1 .. 9 (default 9) -Xwindow-size <window-size> <window-size> should be 8 .. 15 (default 15) -Xstrategy strategy1,strategy2,...,strategyN Compress using strategy1,strategy2,...,strategyN in turn and choose the best compression. Available strategies: default, filtered, huffman_only, run_length_encoded and fixed lzo -Xalgorithm <algorithm> Where <algorithm> is one of: lzo1x_1 lzo1x_1_11 lzo1x_1_12 lzo1x_1_15 lzo1x_999 (default) -Xcompression-level <compression-level> <compression-level> should be 1 .. 9 (default 8) Only applies to lzo1x_999 algorithm lz4 -Xhc Compress using LZ4 High Compression xz -Xbcj filter1,filter2,...,filterN Compress using filter1,filter2,...,filterN in turn (in addition to no filter), and choose the best compression. Available filters: x86, arm, armthumb, powerpc, sparc, ia64 -Xdict-size <dict-size> Use <dict-size> as the XZ dictionary size. The dictionary size can be specified as a percentage of the block size, or as an absolute value. The dictionary size must be less than or equal to the block size and 8192 bytes or larger. It must also be storable in the xz header as either 2^n or as 2^n+2^(n+1). Example dict-sizes are 75%, 50%, 37.5%, 25%, or 32K, 16K, 8K etc. zstd -Xcompression-level <compression-level> <compression-level> should be 1 .. 22 (default 15) lzma (no options) (deprecated - no kernel support) Environment: SOURCE_DATE_EPOCH If set, this is used as the filesystem creation timestamp. Also any file timestamps which are after SOURCE_DATE_EPOCH will be clamped to SOURCE_DATE_EPOCH. See https://reproducible-builds.org/docs/source-date-epoch/ for more information See also: The README for the Squash-tools 4.5.1 release, describing the new features can be read here https://github.com/plougher/squashfs-tools/blob/master/README-4.5.1 The Squashfs-tools USAGE guide can be read here https://github.com/plougher/squashfs-tools/blob/master/USAGE The ACTIONS-README file describing how to use the new actions feature can be read here https://github.com/plougher/squashfs-tools/blob/master/ACTIONS-README ``` </details> ``` ls -lh /media/benjamin_loison/64XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXd8/SSDServer-squash.img ``` ``` -rw-r--r-- 1 benjamin_loison benjamin_loison 84G Sep 12 2022 /media/benjamin_loison/64XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXd8/SSDServer-squash.img ``` ``` ls -l /media/benjamin_loison/64XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXd8/SSDServer-squash.img ``` ``` -rw-r--r-- 1 benjamin_loison benjamin_loison 89841958912 Sep 12 2022 /media/benjamin_loison/64XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXd8/SSDServer-squash.img ``` ``` time sudo mksquashfs /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer{,_squash}.iso ``` <details> <summary>Output:</summary> ``` Parallel mksquashfs: Using 24 processors Creating 4.0 filesystem on /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso, block size 131072. [==========================================================================================================================================================================================|] 1907344/1907344 100%> Exportable Squashfs 4.0 filesystem, gzip compressed, data block size 131072 compressed data, compressed metadata, compressed fragments, compressed xattrs, compressed ids duplicates are removed Filesystem size 67064143.13 Kbytes (65492.33 Mbytes) 27.47% of uncompressed filesystem size (244147484.63 Kbytes) Inode table size 2776566 bytes (2711.49 Kbytes) 36.38% of uncompressed inode table size (7631328 bytes) Directory table size 33 bytes (0.03 Kbytes) 94.29% of uncompressed directory table size (35 bytes) Number of duplicate files found 0 Number of inodes 2 Number of files 1 Number of fragments 0 Number of symbolic links 0 Number of device nodes 0 Number of fifo nodes 0 Number of socket nodes 0 Number of directories 1 Number of hard-links 0 Number of ids (unique uids + gids) 1 Number of uids 1 root (0) Number of gids 1 root (0) real 25m48.123s user 0m0.107s sys 0m0.170s ``` </details> ``` ls -lh /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso ``` ``` -rw-r--r-- 1 root root 64G Aug 3 02:25 /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso ``` ``` ls -l /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso ``` ``` -rw-r--r-- 1 root root 68673683456 Aug 3 02:25 /media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso ``` so it spared 20 GB! DuckDuckGo search *squashfs writable*. [The Unix Stack Exchange answer 80312](https://unix.stackexchange.com/a/80312) is inappropriate in my case as recrompressing is an expensive operation. Related to [Benjamin_Loison/virt-manager/issues/31](https://codeberg.org/Benjamin_Loison/virt-manager/issues/31). DuckDuckGo search *btrfs compress iso*, *compress iso while keeping writable* and *squashfs writable alternative*. https://bbs.archlinux.org/viewtopic.php?id=131678 DuckDuckGo and Google search *btrfs compress file* and *Use btrfs like squashfs*. ``` sudo mkfs.ext4 test.iso ``` <details> <summary>Output:</summary> ``` mke2fs 1.47.0 (5-Feb-2023) The file test.iso does not exist and no size was specified. ``` </details> DuckDuckGo and Google search *Linux create ext4 as file*. [The Unix Stack Exchange question 753111](https://unix.stackexchange.com/q/753111) ``` df -h / ``` <details> <summary>Output:</summary> ``` Filesystem Size Used Avail Use% Mounted on /dev/mapper/pegasus--vg-root 731G 554G 141G 80% / ``` </details> ``` fallocate -l 5G test.iso sudo mkfs.ext4 test.iso ``` <details> <summary>Output:</summary> ``` mke2fs 1.47.0 (5-Feb-2023) Discarding device blocks: done Creating filesystem with 1310720 4k blocks and 327680 inodes Filesystem UUID: 515a90d2-ca70-4939-9292-8f9c4c6fee6e Superblock backups stored on blocks: 32768, 98304, 163840, 229376, 294912, 819200, 884736 Allocating group tables: done Writing inode tables: done Creating journal (16384 blocks): done Writing superblocks and filesystem accounting information: done ``` </details> ``` df -h / ``` <details> <summary>Output:</summary> ``` Filesystem Size Used Avail Use% Mounted on /dev/mapper/pegasus--vg-root 731G 554G 141G 80% / ``` </details> ``` ls -lh test.iso ``` ``` -rw-r--r-- 1 benjamin_loison benjamin_loison 5.0G Aug 3 02:45 test.iso ``` ``` sudo mkfs.btrfs ``` <details> <summary>Output:</summary> ``` btrfs-progs v6.2 See http://btrfs.wiki.kernel.org for more information. usage: mkfs.btrfs [options] <dev> [<dev...>] Create a BTRFS filesystem on a device or multiple devices Allocation profiles: -d|--data PROFILE data profile, raid0, raid1, raid1c3, raid1c4, raid5, raid6, raid10, dup or single -m|--metadata PROFILE metadata profile, values like for data profile -M|--mixed mix metadata and data together Features: --csum TYPE --checksum TYPE checksum algorithm to use, crc32c (default), xxhash, sha256, blake2 -n|--nodesize SIZE size of btree nodes -s|--sectorsize SIZE data block size (may not be mountable by current kernel) -O|--features LIST comma separated list of filesystem features (use '-O list-all' to list features) -R|--runtime-features LIST comma separated list of runtime features (use '-R list-all' to list runtime features) -L|--label LABEL set the filesystem label -U|--uuid UUID specify the filesystem UUID (must be unique) Creation: -b|--byte-count SIZE set size of each device to SIZE (filesystem size is sum of all device sizes) -r|--rootdir DIR copy files from DIR to the image root directory --shrink (with --rootdir) shrink the filled filesystem to minimal size -K|--nodiscard do not perform whole device TRIM -f|--force force overwrite of existing filesystem General: -q|--quiet no messages except errors -v|--verbose increase verbosity level, default is 1 -V|--version print the mkfs.btrfs version and exit --help print this help and exit Deprecated: -l|--leafsize SIZE removed in 6.0, use --nodesize ``` </details> ``` file test.iso ``` ``` test.iso: Linux rev 1.0 ext4 filesystem data, UUID=167c294b-86f3-41ef-9e16-f89a05e535e5 (extents) (64bit) (large files) (huge files) ``` ``` mkdir a/ mount test.iso a/ ``` ``` mount: a/: failed to setup loop device for /home/benjamin_loison/test.iso. ``` ```bash sudo mount test.iso a/ ``` does not return anything. ``` ls -lha a/ ``` <details> <summary>Output:</summary> ``` total 24K drwxr-xr-x 3 root root 4.0K Aug 3 02:48 . drwx--x---+ 44 benjamin_loison benjamin_loison 4.0K Aug 3 02:48 .. drwx------ 2 root root 16K Aug 3 02:48 lost+found ``` </details> ``` umount a/ ``` ``` umount: /home/benjamin_loison/a: must be superuser to unmount. ``` ``` sudo umount a/ ``` does not return anything. ``` sudo mkfs.btrfs test.iso ``` <details> <summary>Output:</summary> ``` btrfs-progs v6.2 See http://btrfs.wiki.kernel.org for more information. NOTE: several default settings have changed in version 5.15, please make sure this does not affect your deployments: - DUP for metadata (-m dup) - enabled no-holes (-O no-holes) - enabled free-space-tree (-R free-space-tree) Label: (null) UUID: b7095905-987f-444b-af34-573aa57d42cd Node size: 16384 Sector size: 4096 Filesystem size: 5.00GiB Block group profiles: Data: single 8.00MiB Metadata: DUP 256.00MiB System: DUP 8.00MiB SSD detected: no Zoned device: no Incompat features: extref, skinny-metadata, no-holes Runtime features: free-space-tree Checksum: crc32c Number of devices: 1 Devices: ID SIZE PATH 1 5.00GiB test.iso ``` </details> ``` sudo mount test.iso a/ ``` does not return anything. ``` ls -lah a/ ``` <details> <summary>Output:</summary> ``` total 20K drwxr-xr-x 1 root root 0 Aug 3 02:51 . drwx--x---+ 44 benjamin_loison benjamin_loison 4.0K Aug 3 02:51 .. ``` </details> DuckDuckGo and Google search *btrfs enable compression*. https://btrfs.readthedocs.io/en/latest/Compression.html I suspect that if the compression is at the file level, then modifying the big file, requires to recompress the file. > Typically the compression can be enabled on the whole filesystem, specified for the mount point. > The command above will start defragmentation of the whole file and apply the compression, regardless of the mount option. With ext4: ``` sudo dd if=/dev/urandom of=a/my_data status=progress ``` <details> <summary>Output:</summary> ``` 2017192960 bytes (2.0 GB, 1.9 GiB) copied, 16 s, 126 MB/s^C 4026336+0 records in 4026335+0 records out 2061483520 bytes (2.1 GB, 1.9 GiB) copied, 16.3456 s, 126 MB/s ``` </details> ``` df -h / ``` <details> <summary>Output:</summary> ``` Filesystem Size Used Avail Use% Mounted on /dev/mapper/pegasus--vg-root 731G 556G 139G 81% / ``` </details> so I notice the 2 GB change. [The Super User answer 1136358](https://superuser.com/a/1136358) may help. ```bash FILE=/media/benjamin_loison/6aXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXce/SSDServer_squash.iso; dd if=$FILE | pv -s `wc -c $FILE | cut -d ' ' -f 1` | dd of=/media/benjamin_loison/64XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXd8/SSDServer-squash_zerofreed.img ``` <details> <summary>Output:</summary> ``` 134128288+0 records iniB/s] [==================================================================================================================================================================> ] 99% ETA 0:00:00 134128288+0 records out 68673683456 bytes (69 GB, 64 GiB) copied, 1344.43 s, 51.1 MB/s 64.0GiB 0:22:24 [48.7MiB/s] [===================================================================================================================================================================>] 100% 134128288+0 records in 134128288+0 records out 68673683456 bytes (69 GB, 64 GiB) copied, 1343.55 s, 51.1 MB/s ``` </details>
Benjamin_Loison commented 2025-08-03 20:30:05 +02:00
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Related to Benjamin_Loison/coreutils/issues/13.

Related to [Benjamin_Loison/coreutils/issues/13](https://codeberg.org/Benjamin_Loison/coreutils/issues/13).
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Reference: Benjamin_Loison/linux#86