[DRBD-user] Re: strange performance problems with drbd

Lars Ellenberg Lars.Ellenberg at linbit.com
Mon May 8 17:55:38 CEST 2006

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

/ 2006-05-08 15:01:08 +0000
\ Torsten Neumann:
> Lars Ellenberg <Lars.Ellenberg at ...> writes:
> > > So how can I check why drbd slows things so terrible down? Yes there is a
> > > bottleneck. But without drbd all seems fine, with drbd not.
> > 
> > see what io scheduler is in use
> > (/sys/block/lower-level-device/queue/scheduler)
> > on servers I prefer "deadline".
> > you can switch to "deadline" like this:
> >   for q in /sys/block/*/queue/scheduler ; do
> > 	echo deadline > "$q"
> >   done
> > or by specifying a boot-time commandline parameter.
> > 
> > if that did not help yet,
> > check wether increasing "max-buffers"
> > and/or decreasing "max-epoch-size" in drbd.conf does help.
> > 
> Neither changer io scheduler from anticipatory to deadline nor lowering
> max-epoch-size showed any effect. max-buffers was on the highest allowed value
> anyway. But decreasing didn't helped also.

would have been a work around anyways.
as noted earlier, and by yourself when using different io-backend:
drbd is able to saturate disk/network bandwidth.

so I consider this a problem of your io-backend, not of drbd.

what I can suggest is: if one kernel version works for you, and an other
does not: try to find out what has changed:
read the kernel change logs, use diff ...

what else can you do, if you really want to try and see if messing with
drbd's tunables could help:
watch the al: and bm: values in /proc/drbd
(they count drbd meta-data updates).
if al: increases very often, you could try and increase al-extents.
there is a tradeof with expected resync time on primary crash, though.
if bm: increases very often, there is not much you can do about it...

one more thought:
are you sure your raid5 does not happen to rebuild itself at the same time?
that would pretty much explain everything...

: Lars Ellenberg                                  Tel +43-1-8178292-0  :
: LINBIT Information Technologies GmbH            Fax +43-1-8178292-82 :
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