UFS Snapshots and iowait

Kurt Lidl lidl at pix.net
Tue Nov 23 19:49:52 UTC 2010


On Tue, Nov 23, 2010 at 10:38:31AM -0800, Chris St Denis wrote:
> Is this just due to the very high io bandwidth usage associated with 
> making a snapshot, or does the creation of this snapshot completely 
> block IO writes for around 5 minutes?

It blocks updates to the filesystem while during part of the
snapshot process.

See the comments in /usr/src/sys/ufs/ffs/ffs_snapshot.c

I found using UFS snapshots on a production fileserver untenable
during normal working hours.  I have a backup fileserver that I
rsync the files to, and then use the UFS snapshots there.

> Any suggested workarounds? I already bumped up the number of Apache 
> slots to 166% but it looks like I would have to increase the number much 
> more to use that as a primary solution.

Use ZFS.  The way snapshots work there, they are nearly instantanous
to create, and you are not limited to 20 snapshots per filesystem.

-Kurt


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