[DRBD-user] Large block IO bottleneck

Philipp Reisner philipp.reisner at linbit.com
Wed Jan 3 11:58:05 CET 2007


Am Dienstag, 2. Januar 2007 22:05 schrieb Ross S. W. Walker:
> Hi there I am using DRBD 0.7.21 with iSCSI Enterprise Target 0.4.14 on
> CentOS 4.4.
>
> When I run iSCSI direct to the LVM lv on top of hardware RAID I can get
> 225 MB/s over two sessions in MPIO with 256K block size, but when I put
> DRBD in-between iSCSI and LVM the throughput tops out at 80 MB/s and I
> can't seem to go over that.
>
> DRBD seems to report it's max number of sectors as 8 (4K), does that
> mean each io operation is limited to 4K? My hardware raid reports it's
> max sectors as 128, could this explain the reduction to 1/3 throughput?
>

Hi,

The cause for the limitation to 4k is the Linux-2.4 compabitility of 
DRBD-0.7.

Repeat your test with drbd-8.0(rc1).

drbd-8.0 will do BIOs up to 32k, but much more important are other
changes (e.g. the non blocking make_request() function), that makes
drbd-8.0 to scale much better with high end hardware.

PS: What kind of network link are you using ?

-Phil
-- 
: Dipl-Ing Philipp Reisner                      Tel +43-1-8178292-50 :
: LINBIT Information Technologies GmbH          Fax +43-1-8178292-82 :
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