On 01/19/10 10:09, krad wrote:
2010/1/18 Morgan Wesstr�m<freebsd-questi...@pp.dyndns.biz>
O. Hartmann wrote:
I realise a strange behaviour of several FreeBSD 8.0-STABLE/amd64 boxes.
All boxes have the most recent STABLE. One box is a UP system, two
others SMP boxes, one with a Q6600 4-core, another XEON with 2x 4-cores
(Dell Poweredge III).
Symptome: All boxes have ZFS and UFS2 filesystems. Since two weeks or
so, sometimes the I/O performance drops massively when doing 'svn
update', 'make world' or even 'make kernel'. It doesn't matter what
memory and how many cpu the box has, it get stuck for several seconds
and freezing. On the UP box, this is sometimes for 10 - 20 seconds.
A very interesting phenomenon is the massively delayed file writing on
ZFS filesystems I realise. Editing a file in 'vi' running on one XTerm
and having in another Xterminal my shell for compiling this file, it
takes sometimes up to 20 seconds to get the file updated after it has
been written. It's like having an old, slow NFS connection with long
cache delays.
These massively delayed file transactions are not necessarely under
heavy load, sometimes they occur in a relaxed situation. They seem to
occur much more often on the UP box than on the SMP boxes, but this
strange phenomenon also occur on the Dell Poweredge II, which has 16GB
RAM and summa summarum 16 cores. This phenomenon does occur on ZFS- and
UFS2 filesystems as well. It is hardly reproducable.
Is there any known issue?
Ragrds,
Oliver
The disks involved don't happen to be Western Digital Green Power disks,
do they? The Intelli-Park function in these disks are wrecking havoc
with I/O in Linux-land at least, causing massive stalls and iowait
through the roof during the 25-30 seconds it takes for the heads to
unload after parking. I have two of these disks sitting on my desk now
collecting dust...
/Morgan
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ZFS is copy on write, therefore to optimize the write performance it delays
writes for a long as possible, upto a set maximum time. It will then flush
to the disks. How long this time is depends on how much free ram you have
available. Assuming processes are eating up all your ram I would imagine you
are hitting the max limit. I'm not sure exactly what its set to on bsd but I
know the default on opensolaris is 30s. I think this explains your delayed
writes.
Not sure what will cause the lock ups though.
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This could end in a bad situation, where one process writes a files, say
with some arbitrary stuff and another successing process is intended to
read this file. even if the processes are run serial, those 'delays'
could break the chain! The delay situation in a development environment
is harsh, but in other circumstances it could develop very bad.
I see this strange behaviour now for several weeks, something essential
has changed in the code, I guess.
On UP boxes the situation is worse sometimes, on SMp boxes with lots of
RAM ( 8 and 16 GB and 4 or 8 CPU cores) it is still bad. I have a server
that acts as a 'rsync' backup system gathering data from satellite
servers from time to time. Since this problem of slowness occured, this
4-core 8 gig RAM box crawls for minutes. Even when X11 is disabled
working on console is 'bumpy': terminal out slows down, mouse pointer
jumps etc.As I wrote, the same on a 8 core/16 gig box, but not that harsh.
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