[DRBD-user] Write performance again

Sebastian Riemer sebastian.riemer at profitbricks.com
Thu Sep 27 11:02:19 CEST 2012

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

Hi Roland,

I guess that you've got a 64k stripe RAID. If IO isn't aligned to the
stripe size it can be slow. The idle HDDs indicate that DRBD is doing
something wrong.
Do you have caching enabled on your RAID controller? A RAID controller
with caching should be able to merge too small IO requests together
before dispatching them to the HDDs.

Furthermore, there is an IO request size limits bug in 8.4.1 as well as
in <= 8.3.13.


Therefore, I really suggest you to trace the block sizes with "blktrace"
to see what's really going on on the block layer.

Here is how you do it:
1. install "blktrace" package - your kernel should support blktracing
2. # blktrace /dev/sdX -b 4096 &
3. # pid=$!
4. # dd ... of=/dev/sdX bs=1M ...
5. # kill -2 $pid
6. # blkparse sdX | less

When parsing you should see something like this:
  8,0    1      177    34.431144943  1275  Q  WS 94666752 + 1024 [dd]

"Q" means IO is queued, "W" it is a write, "+ 1024": 512 KiB were
written to the queue - this is measured in sectors and a sector is 512
Byte long on common HDDs.

Do the tracing on your RAID device without DRBD above first. Then, do
the tracing for your DRBD device.

I really guess that your issue is on the blkio layer. But yes - there
are general network statistics. You could use "iftop" for example. But
when writing only 4 KiB for example on the block layer, then also only 4
KiB are sent through the network layer.


On 26.09.2012 16:18, Roland Kaeser wrote:
> Hello Sebastian
> Thanks for the hint.  The be sure that the settings are exactly the
> same on both sides, I wrote a script to set these settings  by ssh on
> both nodes equally. So the current value for max_sectors_kb is: 64 on
> both nodes. But I played around with these values and nothing helped.
> When the nodes are connected, the write speed is even lower than in
> single node mode. I think there must be some kind of bottleneck
> between the blocksizes / ioscheduler, etc. and drbd but could find the
> igniting hint. 
> Strange is also that the sync speed is very low (triggered manually
> resync to retest that) 66MB /s. All tests with different config
> settings showed no performance changement.  blktrace also shows
> nothing more flashy which could lead to more. while resyncing and
> dd'ing the iostat shows 1.07% iowat which is imho very low value for a
> system under full io load. 
> Is there a possibility to debug the network usage and internal network
> stats of drbd besides drbdsetup show 0?
> Regards
> Roland

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