Messages in this thread | | | Date | Mon, 05 Apr 2004 11:14:39 -0400 | From | Timothy Miller <> | Subject | Re: PROBLEM: Consistently slower 3ware RAID performance under 2.6.4 |
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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.
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