[nSLUG] raid performance (update)

Rory rory at unixism.org
Sat Jun 24 20:50:09 ADT 2006

Thanks to all that replied here or directly to me.

It looks like we found the problem and it has nothing to do with md,scsi
or megaraid.   It's a known bug in 2.6.x libc + reiserfs that causes
reiser to use a 128k read-ahead regardless of the underlying read-ahead
config.  The problem exhibits in fseek() specifically and can be avoided
by ditching ansi fseek() in favour of pread() or using nolargeio=1 when
mounting reiser.

This email thread covers the issue:

For our case we'll start with the mount option and then likely re-write
code later.


Rory wrote:
> I know some folks on this list have experience building heavy lifting
> Linux boxen so here's a quandary:
> Having recently switched to 2.6.15 kernel we are finding that the RAID
> performance completely sucks.  Specifically RAID 10, on Dell 2850 with a
> Perc4 controller.  This is an LSI based controller and therefore uses
> the megaraid driver.  This is initially perplexing given that with the
> new I/O schedulers in 2.6 things should improve.  We've tried all the
> schedulers including the no-op one with no appreciable change in crappy
> performance.
> By crappy performance I mean reads are an order of magnitude slower than
> with 2.4 and writes even slower.
> The part that is really burning our collective noodles is that very
> similar performance problems exist when using software raid with 2.6. 
> Reverting back to the 2.4 kernel on the same hardware returns
> performance to previous both with megaraid and software raid.
> I'm not the one working directly on the problem but since I've been
> asked for my opinions I thought it would be worthwhile to check here and
> see if this rings a bell for anyone before I dive deep into kernel
> debugging mode.
> R
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