[zfs-discuss] "Compression" restriction on root pool sub-datasets?

Swâmi Petaramesh swami at petaramesh.org
Fri May 17 01:38:24 EDT 2013

Le 17/05/2013 02:12, Ned Bass a écrit :
> Grub should be unaffected by compression in any other datasets in your
> pool, since it doesn't need to access those blocks.

But I've seen that lz4 compression inside a pool causes issues with grub
in both Ubuntu raring and ArchLinux, because enablig lz4 compression in
*any* filesystem inside the pool (which includes descendants to be very
specific) needs the pool version to evolve from 28 or less to "feature
flags", and then grub doesn't recognize a ZFS pool with feature flags,
which will make "update-grub" (in ubuntu) or "grub-mkconfig" (in Arch or
other distros) fail miserably with a message stating that it cannot
recognize the FS type for /.

This can be fixed by manually editing a number of files, including
grub-mkconfig and /etc/default/grub, but this is a pain in the a**.

So if you want not to have issues with grub, keep your root pool at
version 28, do *not* enable feature flags on it, which means you can use
compression, but do *not* use lz4 compression anywhere in the pool.

grub can manage gzip compression, and this concerns only the place were
/boot resides, but I would advise not to compress /boot anyway, initrds
are already zipped and /boot is small, so just to stay on the safe side.

Kind regards.

Swâmi Petaramesh <swami at petaramesh.org> http://petaramesh.org PGP 9076E32E

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