Hello
On Tue, 22 May 2007, Thomas Jager wrote:
Hi list.
I run a file server on MD raid-5.
If a client reads one big file and at the same time another client tries to
write a file, the thread writing just sits in uninterruptible sleep until the
reader has finished. Only very small amount of writes get trough while the
reader is still working.
I assume from the vmstat numbers the reader does a lot of seeks (iowait > 80%!).
I'm having some trouble pinpointing the problem.
It's not consistent either sometimes it works as expected both the reader and
writer gets some transactions. On huge reads I've seen the writer blocked for
30-40 minutes without any significant writes happening (Maybe a few
megabytes, of several gigs waiting). It happens with NFS, SMB and FTP, and
local with dd. And seems to be connected to raid-5. This does not happen on
block devices without raid-5. I'm also wondering if it can have anything to
do with loop-aes? I use loop-aes on top of the md, but then again i have not
observed this problem on loop-devices with disk backend. I do know that
loop-aes degrades performance but i didn't think it would do something like
this?
What IO scheduler are you using? Maybe try using a different scheduler
(eg. deadline) if that does make any difference.
Holger
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