linux-kernel - Re: howto boost write(2) performance?

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Date:	Wed, 17 Oct 2007 18:19:07 -0400
From:	Bill Davidsen <davidsen@....com>
To:	linux-kernel@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org
Subject:  Re: howto boost write(2) performance?

Michael Stiller wrote:
> Hi list,
> 
> i'm developing an application (in C) which needs to write about 
> 1Gbit/s (125Mb/s) to a disk array attached via U320 SCSI. 
> It runs on Dual Core 2 Xeons @2Ghz utilizing kernel 2.6.22.7. 

People regularly report speeds higher than that on the RAID list, and I 
can get that order of magnitude speed using dd with 1MB buffers to a 
software RAID-0 array or cheap SATA drives. Are you using a decent 
controller? Many "RAID" controllers have bandwidth limitations, buffer 
size issues, etc. I had some chea "SCSI" arrays which were just SCSI 
controllers in from of a bunch of cheap, slow, non-SCSI drives.
> 
> I buffer the data in (currently 4) 400Mb buffers and use write(2) in a
> dedicated thread to write them to the raw disk (no fs).

I would limit the write size to a MB and see if that helps, regardless 
of the buffer size. A circular queue of smaller buffers, like ptbuf, may 
perform better.

> 
> The write(2) performance is not good enough, the writer threads take to
> much time, and i ask you for ideas, howto to boost the write
> performance. 
> 
> Maybe mmaping the disk would work? 

I don't think it would help, I'd really try limiting the size of the 
write() calls first, assuming your hardware is adequate.

-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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