Re: [linux-lvm] Does LVM have any plan/schedule to support btrfs in fsadm

[Gionatan Danti <g.danti@assyoma.it>]

Il 2021-06-28 05:28 Stuart D Gathman ha scritto:
> Yes.  I like the checksums in metadata feature for enhanced integrity
> checking.

Until recently btrfs has issue when a LVM snapshot was mounted. It is 
now solved?

> It seems too complicated to have anytime soon - but when a filesystem
> detects corruption, and is on an LVM (or md) RAID1 layer, an ioctl to
> read alternate mirror branches to see which (if any) has the correct
> data would allow recovery.  Btrfs does this if it is doing the
> mirroring, but then you lose all the other features from LVM or md
> raid10, including running other filesystems and efficient virtual
> disks for
> virtual machines.

For this to work, LVM should be able to identify the corrupted data. 
Without checksum, how can you do that? The solution is to use 
dm-integrity under the RAID layer. It works pretty well, letting apart 
the big performance drop in the default (journaled) configuration 
(bitmap is faster, but leave a small window for corruption to happen 
undetected).

That said, for rewrite-heavy workload (virtual machines, databases, etc) 
btrfs is very slow (and disabling CoW is not a solution for me, as it 
also disables checksum, compression, etc).

> We eventually got DISCARD operations to pass to lower layers.
> Dealing with mirror branches should really be a thing too.

As said above, the issue is not to read from the mirror leg separately; 
rather, to detect *which* mirror leg contains valid data.

Regards.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
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