[DRBD-user] DRBD and Direct Block IO

Ross S. W. Walker rwalker at medallion.com
Fri Jan 12 02:20:47 CET 2007


> -----Original Message-----
> From: drbd-user-bounces at lists.linbit.com 
> [mailto:drbd-user-bounces at lists.linbit.com] On Behalf Of Ross 
> S. W. Walker
> Sent: Thursday, January 11, 2007 7:48 PM
> To: drbd-user at lists.linbit.com
> Subject: [DRBD-user] DRBD and Direct Block IO
> 
> 
> I am having some serious performance issues with a kernel module for
> iSCSI Enterprise Target and drbd 0.7.21 on CentOS 4.4.
> 
> The kernel module (source code listed below) basically takes a data
> request issued over iSCSI translates it to a bio request that is then
> carried out synchronously to the device below.
> 
> When run against an MD/LVM/SD device I don't have any problems, I get
> performance to be expected, but when run against a drbd 
> 0.7.21 device it
> chokes down hard. For example when doing seq 64K block write direct to
> device I can get 112 MB/s sustained, when I have drbd in the 
> middle that
> throughput drops to 10 MB/s.
> 
> Can anybody help explain this or point out a serious flaw in the code
> below that would cause this. I would rather solve the problem (if it
> can) then try to run version 8 beta in production (if it will 
> even solve
> my problem).
> 
> Thanks,
> 

<snip code sample>

I forgot to mention probably the most important piece of information,
this drbd replica is currently running Standalone with no peer, so there
is actually no replication happening at this time.

-Ross

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