Re: LVM snapshots

On Sun, Mar 16, 2003 at 12:50:58PM -0500, Tom Lane wrote:
> BTW: aside from the question of the integrity of the snapshot backup,
> I'd be inclined to ask hard questions about the effects on database
> performance from using the disk-freezing feature in the first place.
> What is the LVM layer doing with the writes that occur while the
> frozen snapshot is maintained??  If they're not getting to disk at
> all, then "commits" that occur during the freeze aren't really commits
> (and one can be pardoned for wondering whether write ordering
> constraints will be honored when the floodgate is reopened).  If they
> are getting to disk, this seems to imply data journaling is being used,
> which is probably a net loss for overall database performance.  You
> might be willing to pay that cost to be able to use this backup
> technique --- but you should find out what that cost is ...

All sync writes will hang while the snapshot is beeing taken. Not sure
if async writes are buffered in memory or will hang too.

So, basicly your database will be inresponsive for the time it takes to
take a snapshot. The time will depend on the size of the device, the
size of the pysical extents (the LVM "blocks") and the raw disk
performance, but typically we're talking about periods shorter than a
second.


--
Ragnar KjÞrstad
Zet.no