Holger Kiehl wrote:
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 don't think so unless the file is really fragmented. But I doubt it.
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.
I was using deadline. I tried switching to CFQ but I'm still seeing the
same strange problems.
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