Date: Wed, 26 Mar 2008 09:05:27 +0100
From: Emmanuel Florac <eflorac@intellique.com>
To: Bill Davidsen <davidsen@tmr.com>
Cc: Chris Snook <csnook@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: RAID-1 performance under 2.4 and 2.6
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Le Tue, 25 Mar 2008 19:42:28 -0400 vous ?criviez:

> And this is what I was saying earlier, there is a trend to blame the 
> benchmark when in fact the same benchmark runs well on 2.4. 

As I mentioned, it looks like 2.4 actually buffers write data on RAID-1
which is inherently bad (after all if I do RAID-1 it's for the sake of
data integrity, and write caching just counters that).
However, how bad dd may be, it reflects broadly my problem : on small
systems using software RAID, IO is overall way better with 2.4 than
2.6, especially NFS thruput.
Though I can substantially enhance 2.6 performance through tweaking
(playing with read ahead, disk queue length etc), it still lags behind
2.4 with defaults settings by a clear margin (10% or more).
This isn't true - fortunately - of larger systems with 12, 24, 48 disks
drives, hardware RAID, Fibre Channel and al. 

-- 
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Emmanuel Florac               www.intellique.com   
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