Re: PROBLEM: Consistently slower 3ware RAID performance under 2.6.4

[Timothy Miller <miller@techsource.com>]

Andrew Morton wrote:

> 
> You cannot compare 2.4 and 2.6 kernel write performance with `dd', because
> the kernels are tuned differently.  2.6 kernels are tuned to leave less
> dirty pages in memory than a 2.4 kernel.  Hence when your dd has finished,
> 40% of memory will be dirty (needing writeout) under 2.6, but this figure
> is 60% on 2.4.
> 
> So the 2.6 kernel does more writeout during the dd run and less writeout
> after dd has finished.  The 2.4 kernel does less writeout during the dd run
> and more writeout after dd has finished.
> 
> To compare IO performance you'll need to set 2.6's /proc/sys/vm/dirty_ratio
> to 60 and /proc/sys/vm/dirty_async_ratio to 30.  Or use write-and-fsync
> from http://www.zip.com.au/~akpm/linux/patches/stuff/ext3-tools.tar.gz with
> the `-f' option.
> 
> I don't know why the read bandwidth appears to be lower.  Try increasing
> the readahead with `blockdev --setra'?


That's odd.  I've tried different mem= kernel parameters with no change 
in throughput under 2.4.  That is, 1G vs. 128M vs. 32M all perform the 
same under 2.4 when doing dd to a block device.  On the other hand, 
available RAM makes a HUGE difference when doing dd to a _file_system_.

I haven't tried that under 2.6 yet.

Anyhow, I chose a 1GB test run so as diminish cache effects and file 
system overhead.  Is my logic flawed here?

Thanks.