On Wed, 21 May 2008 12:24:01 +0300, Ira Abramov wrote:
>
> problem is that the CAD tools running here dictate that we stick to
> RHEL4 (damn you, Cadence! how hard is it to support RHEL5?!), and
> snapshots are problematic at 2.6.9 (some problems were solved only at
> 2.6.16)
>
> > > The more I read about ZumaStor I like it more, but the big boon in LVM2
> > > is that I don't have to recompile RHEL's kernel. so any production-env
> > > experiance shared will be appreciated.
> >
> > That's a big advantage if you ask me.
> > What's a killer argument against LVM2?
>
> instability in the old kernels, multiple snapshots mean multiple COWs
> (which zumastor is built to solve). people here wanted hourly0,1,2 and
> daily0,1,2 AND weekly0,1,2. that means 9 growing snapshots at any given
> moment, and that is one very serious disk handicap. snapshots in LVM2 are
> not really designed for that, they were mainly meant to be for hot
> backups of file systems, so you can freeze the FS just long enough to
> snap the state to a tarball and remove the snapshot.
You don't want many snapshots because of the heavy load penalty (each
write is multiplied by the number of snapshots).
If you have enough disk space you can create "pseudo" snapshots by
using hardlinks between the snapshots so only changed file takes space.
It really depends on the file size distribution - few large files that
change constantly will defeat this concept, while many small files is
ideal for this scenario.
I use this approach and keep more than 60 "snapshots" of 350 GB disk
on a 700 GB disk.
The whole process is very simple:
Initiation: copy the whole disk (or an LVM snapshot of it) to the
destination disk in a SUBDIRECTORY (eg `cp -a').
For each snapshot:
1. Check the size left on the "snapshot" disk and remove old
"snapshot"s until you have predefined minimum space.
2. Copy the subdir (hardlink only) to a new subdir (I use `cp -al').
3. Create an LVM snapshot of the original system.
4. Sync the copied subdir with the real LVM snapshot (`rsync').
5. Remove the LVM snapshot.
I can send you a script we use on our system but most of it is our
specific details.
Ehud
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