LVM commands extremely slow during raid check/resync

[Larkin Lowrey <llowrey@nuclearwinter.com>]

I've been suffering from an extreme slowdown of the various lvm commands
during high I/O load ever since updating from Fedora 15 to 16.

I notice this particularly Sunday AMs when Fedora kicks of a raid-check.
What is normally near instantaneous, commands like lvs and lvcreate
--snapshot take minutes to complete (literally). This causes my backup
jobs to timeout and fail.

While all this is going on, the various filesystems are reasonably
responsive (considering the raid-check is running) and I can read/write
to files without problems. It seems that this slow-down is unique to lvm.

I have three raid 5 arrays of 8, 6, and 6 drives. The root fs sits
entirely within the 8 disk array as does the spare area used for snapshots.

Interestingly, perhaps, if I can coax a backup into running, the lvs
command, for example, will complete in just 15-30 seconds instead of
120-180s. It would seem that the random I/O of the backup is able to
break things up enough for the lvm commands to squeeze in.

I'm at a loss for what to do about this or what data to scan for clues.
Any suggestions?

kernel 3.2.10-3.fc16.x86_64

lvm> version
  LVM version:     2.02.86(2) (2011-07-08)
  Library version: 1.02.65 (2011-07-08)
  Driver version:  4.22.0

--Larkin

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