[DRBD-user] Some interesting metadata stuff

Lars Ellenberg lars.ellenberg at linbit.com
Sat May 9 12:14:03 CEST 2009

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


On Fri, May 08, 2009 at 06:27:36PM -0400, Gennadiy Nerubayev wrote:
> Possibly related to my earlier post, but this time I focused on two things
> primarily: SSDs and metadata.
> 
> Hardware: DRBD on top of 1 X25-e in each of the DRBD nodes, configured as
> simple volume on a hardware raid controller, and connected via IP over
> infiniband.
> Workload: 100% random io writes, 8KB block size
> 
> Direct to the disk: 77.78MB/s, average latency 2ms
> Disconnected DRBD, metadata on ramdisk: 75.64MB/s, average latency 2ms
> Connected DRBD, metadata on ramdisk: 50.87MB/s, average latency 4ms
> Disconnected DRBD, metadata internal: 6.25MB/s, average latency 39ms
> Connected DRBD, metadata internal: 6.20MB/s, average latency 39ms
> Disconnected DRBD, metadata on a single 15K sas disk: 43.46MB/s, average
> latency 5ms
> Connected DRBD, metadata on a single 15K sas disk: 39.32MB/s, average
> latency 5ms

Could you add a "dm linear" to the table?
i.e. just checking if just one small layer of "virtual" block device
has any effect on throughput and or latency.
dmsetup create experiment <<<"0 $[20 <<21] linear /dev/sdX"
then do your benchmark against /dev/mapper/experiment
(just to see if that performs any different than "raw" /dev/sdx)

> Full resync speeds for all tests were 180-200MB/s (about what's expected
> from a single x25-e). There is no difference between flexible and regular
> metadata for internal or external usage (metadata was recreated for those
> tests). Interestingly, ~6MBs is the same speed that I got when testing a 7
> disk raid 0 15K sas array with internal metadata (everything else is the
> same) and putting metadata on ramdisk moved that up to ~35MB/s.
> 
> So for some reason even in the case of a very fast SSD, internal metadata
> performance for random writes is really bad. Putting it on any kind of
> external disk brings an immediate exponential performance increase..

IO scheduler? try deadline and noop.
echo deadline  > /sys/block/sdX/queue/scheduler

thanks for feedback.

-- 
: Lars Ellenberg                
: LINBIT HA-Solutions GmbH
: DRBD®/HA support and consulting    http://www.linbit.com

DRBD® and LINBIT® are registered trademarks of LINBIT, Austria.
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