[DRBD-user] [OT] rsync issues [Was Re: Read performance?]

Lars Ellenberg lars.ellenberg at linbit.com
Tue Jun 5 16:54:54 CEST 2007

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

On Tue, Jun 05, 2007 at 08:30:12AM -0500, David Masover wrote:
> On Tuesday 05 June 2007 04:12:12 Lars Ellenberg wrote:
> > tune the io scheduler(s).
> > get the queue length down.
> Ok, which scheduler, and where? I ask because I see tunable things like:
> /sys/block/sda/queue/scheduler
> /sys/block/sda/queue/iosched
> But the "queue" directory doesn't even exist for DRBD:

that is what I'm saying. drbd has no "reqeust queue" in that sense.
(dm-linear has neither, btw.,
to draw an analogy between two "virtual" block device drivers...)
drbd just does some housekeeping and then passes the requests along.

that's why it is very unlikely that it would be drbd's fault
when your reads take too long.

have a look at your lower level devices.

generally I recommend deadline.
for your situation (high latency network link) I suggest small nr_requests.
whether or not that is the real problem, I cannot tell, this has only
been an educated guess.

so I also strongly recommend that you do not just tune blindly
something somewhere, because someone (me) told you (based on *guessing*
where the problem may be) that this *might* have an effetc...
and then it does not help, and you complain that it had no effect...

first get the facts.
monitor your io subsystem (maybe iostat helps?),
monitor the counters in /proc/drbd,
find out what is really going on.

until you know what is going on, you cannot really tune.

: Lars Ellenberg                                  Tel +43-1-8178292-0  :
: LINBIT Information Technologies GmbH            Fax +43-1-8178292-82 :
: Schoenbrunner Str. 244, A-1120 Vienna/Europe   http://www.linbit.com :

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