We have a 2TB LV and create a 32GB snapshot and then modify a few GB
on the LV. The snapshot never fills up and the writes finish fairly
quickly. The problem occurs when we try to remove the snapshot
immediately after completing the file modifications. The lvremove
takes a long time to complete, sometimes a couple of hours. The disk
raid that backs the LV is capable of 300MB/s, so writing everything
should not take that long. I have run some performance tests and
determined that write performance is only about 25% of the raw
throughput available on the disks, but that doesn't explain the really
long time required to finish the lvremove.
The lvremove is not really doing anything -- the backtrace doesn't
change and lvremove is in an uninterruptible sleep. During this time,
iowait time (via iostat) goes up as far as 75% (average of 8 cpus).
The lvremove has always completed and the slow behavior is very
repeatable. There does appear to be a threshold beyond which the slow
behavior occurs. If only a small amount of data is written, then the
lvremove completes quickly. Also, if I wait for a long period of time
(the caches sync?), then the lvremove completes quickly.
We are running 2.6.9-67.0.22.EL and the LVM2 version is
lvm2-2.02.37-3.el4.
I've looked through recent linux-lvm archives and haven't seen
anything that specifically addresses this issue.
Any clues or even pointers of where to look? Have changes been made
in later kernels to address this issue?
Thanks,
Wayne
--
Wayne H. Badger
badger@yahoo-inc.com
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