Re: [Xen-users] Yet another backup proposal (file based vbd's & lvm)

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

On Wednesday 18 October 2006 00:11, Roger Lucas wrote:
> > >From what I've gathered in the list archives and google, LVM snapshots
> >
> > are frowned upon, rdiff-backup seems to get decent remarks, and
> > dd-like backups are too slow and space consuming.
>
> ?? LVM frowned upon ??
>
> Can you clarify this - we use LVM here with Xen-3.0.2 and it works well. 
> There are a lot of notes and discussions of using LVM as a block device for
> the DomU's.  AFAIK, Xen+LVM is pretty widely used.

LVM snapshots are frowned upon because if you keep them running for a long 
time (e.g. to emulate a copy-on-write block device) they tend to chew up lots 
of memory, and eventually cause out of memory conditions in dom0 - bad!

Using them to snapshot during a backup is probably less of a problem that 
these long-lived snapshots.

Various folks have been looking at alternatives for long-lived snapshots - the 
blktap approach supports a few, for instance.

Cheers,
Mark

> > What I'd like to attempt to setup (based on the feedback I receive) is
> > the following.
> >
> > 1. Pause the domU using "xm pause"
> > 2. Sync the domU using "xm sysrq"
> > 3. Use rdiff-backup to make a local backup of the file VBD
> > 4. Resume the domU using "xm unpause"
>
> If you don't exclude LVM and instead use it to provide LVs as VBDs for the
> DomUs, then an alternative sequence of events would allow you to resume the
> DomU a lot faster:
>
> 1. Pause the domU using "xm pause"
> 2. Sync the domU using "xm sysrq"
> 3. Use LVM to snapshot to LV providing the VBD to the DomU
> 4. Resume the domU using "xm unpause"
> 5. Use rdiff-backup to make a local backup of the snapshot LV
> 6. Remove the snapshot
>
> Moving the rdiff-backup from stage 3 to stage 5 will allow you to reduce
> the time between the pause and unpause actions.  Running rdiff-backup on
> (for example) a 10GB block device image will still take some time even on a
> fast system (e.g. with 200MB/sec disk access, just scanning the block
> device for data changes will take more than 100 seconds, 50 secs to read
> the old backup image and 50 secs to read the new file VBD), so you probably
> wouldn't want to have to freeze a running server for this long.  In
> comparison, taking a snapshot of a LV should take less than a second, after
> which you can resume the server again.
>
> BR,
>
> Roger
>
>
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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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