You was right: turning off softdep made the freezes much shorter.
Thanks.
On 01/06/13 13:49, Stefan Sperling wrote:
On Sun, Jan 06, 2013 at 12:22:44PM +0100, Federico Giannici wrote:
We have an OpenBSD 5.2 amd64 where every 5 minutes a few thousand of
".rrd" files from MRTG are written (actually, updated) to disk.
The problem is that for a few seconds (15-20) every other access to
the disk is totally blocked. So during those 15-20 seconds the
access to the graphs is freezed! And this is really annoying for a
graphs server...
It's not a problem of CPU load (it's a quadruple core AMD Athlon II
X4 630 Processor). Processes run smoothly, they freeze only when
they try to access the disk. Disk is a normal SATA, and the
partition is FFS with softdep.
It's probably the bug in the buffer cache where the kernel would allow
userland to queue up so many writes that eventually the kernel is starved
out of buffers. Everything else (for example, read operations on your
graph files) then sleeps until enough writes have been spilled out to disk.
Is there anything (some system tuning?) I can do to get rid of the
freezes, or at least to mitigate them?
The best solution is an upgrade to -current where this has been fixed.
See http://marc.info/?l=openbsd-cvs&m=135231065926430&w=2 and other
related commits by Bob Beck.
If you'd rather stick to 5.2 you can try turning off softdep. softdep delays
some write operations so turning it off might help somewhat by allowing more
read operations to interleave with write operations. While the bug was
affecting -current I found that my systems where much more responsive with
softdep turned off.
