[DRBD-user] Online Verify Seems Really Slow

[Shaun Thomas]

On 06/01/2012 10:37 AM, Lionel Sausin wrote:

> Looks like you're using adaptative syncer rate, so verify will be
> throttled when applications request I/O.

Which isn't the problem here. These are two completely idle boxes set 
aside specifically for testing. My iostat results on the underlying 
device show DRBD is flooding the IO channel with as many reads as it can 
handle (7000-8000 per second). CPU is also very low, no more than 20% on 
each side. Sar says io wait is over 10%, which is actually pretty high.

But like I said, in our stage and production systems which have much 
more capable RAID1+0's and a similarly speedy FusionIO ioDrive, we don't 
notice this. Partially because these kinds of devices can handle random 
IO much better than a 2-disk RAID-0. But my question: why is DRBD doing 
random IO on the verify? If it's following the map to the disk, that's a 
seek for each map entry. I would have thought it would take a gulp of 
several sequential blocks, find the map entries, calculate the MD5's, 
and then compare those across the network.

 From observation, that doesn't seem to be what's happening on either 
8.3 or 8.4.

-- 
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-444-8534
sthomas at optionshouse.com

______________________________________________

See http://www.peak6.com/email_disclaimer/ for terms and conditions related to this email