[DRBD-user] Odd observation when sync'ing

[Lee Christie]

Well yes clearly 300MB/s is the maximum writes, however my point was
that two of the drbd resources were sync'ing very slowly compared to the
others. When we looked at top, we noticed these two down the bottom of
the receiver processes. Using renice I bumped them up and they
increased.

So we suspected CPU contention in as much that the writes weren't evenly
spread.

Now they are spread evenly, even using a single CPU, but we've somehow
lost a whole load of performance.


On the first run I also found (using sar) that both system and iowait
were quite high (>40%) and the system load on the box doing the writes
was over 6 ! it was a box under high IO make no mistake.

However now on a resync the %system is 15%, there is no iowait and the
load is 0.5.


So definitely, something in the first scenario made the storage
subsystem go as fast and hard as it could, and now its not. Baffling.

> -----Original Message-----
> From: drbd-user-bounces at lists.linbit.com 
> [mailto:drbd-user-bounces at lists.linbit.com] On Behalf Of Florian Haas
> Sent: 03 July 2008 15:12
> To: drbd-user at lists.linbit.com
> Subject: Re: [DRBD-user] Odd observation when sync'ing
> 
> Lee Christie wrote:
> > Whats even odder is that looking at top we suspected CPU 
> contention (the
> > boxes have a single CPU each)
> 
> CPU contention? You got a cumulative syncer rate of over 300 MB/s, and
> your disk stats confirmed this. Since I doubt your storage is 
> capable of
> pulling more than 300 MB/s streaming writes, it seems like you simply
> saturated your storage bandwidth, doesn't it?
> 
> Florian
> 
> -- 
> : Florian G. Haas
> : LINBIT Information Technologies GmbH
> : Vivenotgasse 48, A-1120 Vienna, Austria
> 
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