Re: BTRFS as image store for KVM?

[Gert Menke <gert@menke.ac>]

Hi,

thank you for your answers!

So it seems there are several suboptimal alternatives here...

MD+LVM is very close to what I want, but md has no way to cope with 
silent data corruption. So if I'd want to use a guest filesystem that 
has no checksums either, I'm out of luck.
I'm honestly a bit confused here - isn't checksumming one of the most 
obvious things to want in a software RAID setup? Is it a feature that 
might appear in the future? Maybe I should talk to the md guys...

BTRFS looks really nice feature-wise, but is not (yet) optimized for my 
use-case I guess. Disabling COW would certainly help, but I don't want 
to lose the data checksums. Is nodatacowbutkeepdatachecksums a feature 
that might turn up in the future?

Maybe ZFS is the best choice for my scenario. At least, it seems to work 
fine for Joyent - their SmartOS virtualization OS is essentially Illumos 
(Solaris) with ZFS, and KVM ported from Linux.
Since ZFS supports "Volumes" (virtual block devices inside a ZPool), I 
suspect these are probably optimized to be used for VM images (i.e. do 
as little COW as possible). Of course, snapshots will always degrade 
performance to a degree.

However, there are some drawbacks to ZFS:
- It's less flexible, especially when it comes to reconfiguration of 
disk arrays. Add or remove a disk to/from a RaidZ and rebalance, that 
would be just awesome. It's possible in BTRFS, but not ZFS. :-(
- The not-so-good integration of the fs cache, at least on Linux. I 
don't know if this is really an issue, though. Actually, I imagine it's 
more of an issue for guest systems, because it probably breaks memory 
ballooning. (?)

So it seems there are two options for me:
1. Go with ZFS for now, until BTRFS finds a better way to handle disk 
images, or until md gets data checksums.
2. Buy a bunch of SSDs for VM disk images and use spinning disks for 
data storage only. In that case, BTRFS should probably do fine.

Any comments on that? Am I missing something?

Thanks!
Gert