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Re: [linux-lvm] LVM and snapshot performance

On Jan 16, 2009, at 4:36 PM, Alasdair G Kergon wrote:
1. You don't say what filesystem/journal configuration is involved.

We're using an ext2 fs. I have tried with ext3 with journaling and I get the same result.

2. Is it a system where you can run test kernels or is it too 'live' for that?

I can run tests.

3. Try all the syncing and cache flushing options before issuing the lvremove.

I drop caches before running the test. A sync immediately after writing the files results in the same hang.

4. Could it depend on the amount of io load over a short period before the
lvremove runs, rather than something cumulative?

I do notice that smaller data writes result in a quicker lvremove, one that is commensurate with the amount of data written. It seems as though there is a threshold over which the slow result occurs. Also, waiting a long time after the writes complete clears the problem and the lvremove occurs quickly. It seems to be related to a burst of data of a certain size.

5. What more can you find out from ps/iostat/sysrq/lvm logging about what the
system is doing during the lvremove?

iostat shows that there is not a whole lot of I/O going on. There are writes going on pretty continually during the slowdown time, but the disk is certainly not pegged.

ps shows that the lvremove is stuck in an uninterruptible sleep.

6. What devices/drivers are involved in the stack? (We had a perhaps-related
issue show up with loop.)

We're using an HP P400 smart array controller with RAID5. There are 4 750GB SATA disks hanging off the controller.


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