Re: [Xen-users] Xen, LVM and snapshots

[Tomasz Chmielewski <mangoo@xxxxxxxx>]

Morten W. Petersen schrieb:

(...)

> > Newer kernels (2.6.20, maybe even 2.6.19, and later) are not affected by
> > this "phenomenon".
> >
> > Another LVM disadvantage is that every snapshot means additional writes.
> > I.e. if you have a logical volume, and 4 snapshots, writing to the
> > origin will mean 4 more writes are needed. This is a serious scalability
> > problem.
> Well, what I was thinking, was to do a sync on the VM, and flush the
> MySQL database and then immediately afterwards, taking a snapshot of the
> system, backup that snapshot and then after backup "releasing"
> (deleting) the snapshot.
>
> We're stuck with .18 for now, as that's what Debian comes with and we
> try to stick with off-the-shelf packages.
>
> Do you think that's feasible?
Personally, I don't think taking snapshots is a good backup strategy, 
for several reasons:
- they take a lot of space
- the more snapshots you have, the more I/O load you will have (writes 
will go to every snapshot)
- unless you're running a bleeding edge kernel with experimental 
patches, there is no way to turn the snapshot into original (other than 
copying it with dd, which will fill the snapshot, which means the 
snapshot has to be 100% of the size of the origin; which takes a lot of 
space)
- for most Linux systems, taking backup is as easy as copying all files 
even from a live system; for databases, dump is of course needed to 
guarantee consistence; restoring backup is as easy as copying the files 
back - done
- you really should make backups to a different machine, preferably 
somewhere else

That said, I use snapshots for:
- Windows - it's a pain in the a*** to restore a working Windows machine 
if you only have file backups; I usually keep snapshots for the last 3 
days here (in addition to the usual file backup)
- if I make any major changes, and don't have time to do a full file 
backup to another machine

For backup, I can recommend BackupPC.


--
Tomasz Chmielewski
http://wpkg.org

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