Re: [Xen-users] A simple backup

[Javier Guerra Giraldez <javier@xxxxxxxxxxx>]

On Friday 09 May 2008, Nick Anderson wrote:
> Fajar A. Nugraha wrote:
> > IMHO, even the best commercial SAN-based backup solution uses that
> > assumption, so for practical purposes, atomic backup for domU's storage
> > should be sufficient.
>
> So are local logical volume snapshots atomic? I could see a network
> block device not being quite atomic with lv snapshot, but it seems that
> local storage on lv should be atomic. I keep reading conflicting data
> about weather it is or is not atomic.

yes they are.

Fajar's point is that if you take a snapshot of a running VM's storage, it 
reproduces that storage at a point in time where it was mounted and being 
used.  if later you use that storage without the rest of that VM's state (the 
RAM and CPU state), it's just like you had a hard reset, and have to do some 
fsck.

some filesystems are more resilient to that kind of 'failure'; i think ext3 is 
among the best on that regard, I think some BSD's can be even better.  (i 
have no idea how ZFS does, but Fajar implies that it's really good).

curiously, one of the worse is XFS.  even if it's one of the more stable 
filesystems out there, it heavily assumes that it won't _ever_ get a 'yank of 
the cord'.

-- 
Javier

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