Squashfs

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Squashfs is a compressed read only file system. It supports gzip, lzma and lzo compression. It can be used together with aufs (manual) to provide a fast writable filesystem for desktop and embedded systems.

Gentoo Linux users can use squashfs+aufs to create a fast root filesystem.[1]

[edit] mksquashfs

Filesystems are made using mksquashfs. -b specifies the block size, 131072 (128k) is the default size. Larger block size usually results slightly in better compression, but the read speed will be worse. The default block size is a good choice.

Shell command:
nice -n 19 mksquashfs /usr /squashed/backup/usr.sfs.`date --iso` -comp xz -b 131072

xz is slower when making and compressing the filesystem. xz compresses better and read performance will therefore be better depending on harddrive speed and CPU.

[edit] Compression comparison

These are the results of a /usr tree on a very big Gentoo Linux installation with a few games being compressed using gzip and xz.

19090 Mbytes filesystem

gzip, 128k bs

gzip, 256k bs

xz, 128k bs

xz, 256k bs

Compressed size

8992.04 Mbytes

8955.73 Mbytes

8384.18 Mbytes

8247.87 Mbytes

% of uncompressed filesystem size

48.23%

48.04%

44.97%

44.24%

% uncompressed inode table size

28.59%

28.54%

22.32%

22.17%

% uncompressed dir size

36.37%

36.26%

33.46%

33.40%

The difference between xz and gzip is huge, the difference between 128k and 256k block sizes is not.

[edit] References

  1. Gentoo Forums :: View topic - Root partition as SquashFS + Aufs2
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