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[linux-lvm] LVM and snapshot performance



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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