[DRBD-user] Large block IO bottleneck

Ross S. W. Walker rwalker at medallion.com
Fri Jan 12 04:10:27 CET 2007

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

I solved this problem. It turns out it wasn't drbd at all in this case.
The ethernet drivers on the clients doing the benchmarking were, for a
lack of a better term, wacky. I upgraded the drivers and I am getting
consistently better results now.

I have discovered a separate-but-related problem though with a different
back-end io driver for iSCSI that attempts to do direct synchronous io
to the drbd device. When using this back-end module to directly access
an LVM/MD/SD device the throughput is fine, but when run on drbd the
performance is horrible.

I posted a separate post under "DRBD and Direct Block IO" to the list
and included the source code of the back-end module that does the block
io. Maybe there is something in the way it does the io that is
incompatible with DRBD.


> -----Original Message-----
> From: drbd-user-bounces at lists.linbit.com 
> [mailto:drbd-user-bounces at lists.linbit.com] On Behalf Of 
> Leroy van Logchem
> Sent: Thursday, January 04, 2007 3:58 AM
> To: drbd-user at lists.linbit.com
> Subject: Re: [DRBD-user] Large block IO bottleneck
> >>>> Hi there I am using DRBD 0.7.21 with iSCSI Enterprise
> >>>>         
> > The 32k are a rather arbitrary value we have chosen in 
> DRBD. Such a limit
> > is needed for some of the algorithms in the area of the two primary
> > nodes, write conflict detection code.
> >
> > If you really want you can change that define (HT_SHIFT), 
> but probably
> > the positive effects are out-numbered by negative effects 
> (more collision
> > in hash tables etc...)
> >   
> I was hoping to analyse the difference by patching the source but the 
> HT_SHIFT define
> is only used in the 8.x code. What would need changing to try 
> 32K BIO's 
> in the 0.7.x code?
> _______________________________________________
> drbd-user mailing list
> drbd-user at lists.linbit.com
> http://lists.linbit.com/mailman/listinfo/drbd-user

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