[ic] 'Unable to send mail' - ongoing issue

Mike Heins mike at perusion.com
Wed Mar 2 17:08:42 EST 2005

Quoting Daniel Browning (db at kavod.com):
> * Mike Heins <mike at perusion.com> [2005-03-02 12:41]:
> > Quoting Daniel Browning (db at kavod.com):
> > > I periodically get this problem on one catalog.  I have been running PreFork
> > > with PERL_SIGNALS=unsafe the whole time.  It occurs during heavy system load.
> > > This is on a Red Hat Enterprise 3 Dual 2 ghz with 2GB ram and a fast RAID. Perl
> > > 5.8.3, 5.8.4, and 5.8.5 all seem to experience the issue.  It has occurred about
> > > 1-2 times per month for the last six months.
> > 
> > If your system is RAID for everything, even for sessions, you are
> > probably running into disk latency problems. Don't believe the hogwash
> > the RAID controller makers will tell you, that their controllers are
> > faster for write. I have tested it many times, and with frequent writes
> > I am always able to demonstrate that eventually you will get the RAID
> > controller to have a significant slippage causing slowdown.
> > 
> > Your best bet is to have a non-RAID disk and use that for your session
> > storage.
> That's a good idea, maybe I'll do that.  (You've actually suggested that to me
> in the past, and I've even done it on a different system, but I've forgotten all
> about it...).  
> I didn't mention it before, but the system is using RAID-5.  RAID-5 can
> never be as fast as a non-RAID as writes (except for the very short time
> that it hits the write cache), but a RAID-10 system should theoretically be
> twice as fast on writes.

In theory, there is no difference between theory and practice. But, in
practice, there is.

All of these RAID controllers rely on tagged queueing to dispatch their
writes in such a fashion as to keep up with the flow. All you have to
do is overflow their queue, and they are history.

The are definitely faster writing big blocks of data. The problem is,
sessions are these tiny 5-30K files which don't even begin to reach
the point where you get a gain.

Mike Heins
Perusion -- Expert Interchange Consulting    http://www.perusion.com/
phone +1.765.647.1295  tollfree 800-949-1889 <mike at perusion.com>

There's nothing sweeter than life nor more precious than time.
-- Barney

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