UFS Snapshots and iowait

Chris St Denis chris at smartt.com
Wed Nov 10 18:33:43 UTC 2010

I have started using "mount -u -o snapshot" as part of my backup process 
in order to have a week worth of local differential backups to allow 
quick and easy recovery of lost/overwritten/etc files.

The snapshot of the partition (~250G and 2.3 million inodes used. ~10GB 
of data change per day) takes around 10 minutes to complete. During the 
first 5 minutes everything seems to be find, but during the second 5 
minutes the Apache processes that are logging to this drive start 
building up in L (logging) state until they hit MaxClients.

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?

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.

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