Re: ANNOUNCEMENT: Squashfs released (a highly compressed filesystem)

Phillip Lougher
2002-10-30T23:55:09Z

Jeff Garzik wrote:
> Phillip Lougher wrote:

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>> 2. Squashfs compresses inode and directory information in addition
>> to file data. Inodes/directories generally compress down to 50%, or
>> say on average 8 bytes or less per inode.
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> squashfs or mksquashfs?

mksquashfs...

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> A r/w compressed filesystem would be darned useful too :)
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> Jeff
>

A r/w compressed filesystem may be my next project... As a couple of
people have mentioned there are compressed r/w filesystems already
out there.

As you'll know, there are always tradeoffs with filesystem design,
it is very difficult to get as good compression with a r/w fs
than with a read only filesystem. I wanted to get maximum
compression, and quite a few of the techniques I use rely on
its read-only nature.

An append only (i.e. files can be added, but not modified), fs might
be a useful compromise. With compressed metadata, any modification
of files will inevitably achive different compression ratios, and so
modification of metadata/files in place is not an option. Appending
modified metadata/data brings you to log-structured (journalling)
filesystems and compaction (log cleaning) requirements with consequent
loss of compression.

Phillip

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