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SubjectRe: RAID-1 performance under 2.4 and 2.6
On Tue, Mar 25, 2008 at 11:00 PM, Chris Snook <csnook@redhat.com> wrote:
> Emmanuel Florac wrote:
> > I post there because I couldn't find any information about this
> > elsewhere : on the same hardware ( Athlon X2 3500+, 512MB RAM, 2x400 GB
> > Hitachi SATA2 hard drives ) the 2.4 Linux software RAID-1 (tested 2.4.32
> > and 2.4.36.2, slightly patched to recognize the hardware :p) is way
> > faster than 2.6 ( tested 2.6.17.13, 2.6.18.8, 2.6.22.16, 2.6.24.3)
> > especially for writes. I actually made the test on several different
> > machines (same hard drives though) and it remained consistent across
> > the board, with /mountpoint a software RAID-1.
> > Actually checking disk activity with iostat or vmstat shows clearly a
> > cache effect much more pronounced on 2.4 (i.e. writing goes on much
> > longer in the background) but it doesn't really account for the
> > difference. I've also tested it thru NFS from another machine (Giga
> > ethernet network):
> >
> > dd if=/dev/zero of=/mountpoint/testfile bs=1M count=1024
> >
> > kernel 2.4 2.6 2.4 thru NFS 2.6 thru NFS
> >
> > write 90 MB/s 65 MB/s 70 MB/s 45 MB/s
> > read 90 MB/s 80 MB/s 75 MB/s 65 MB/s
> >
> > Duh. That's terrible. Does it mean I should stick to (heavily
> > patched...) 2.4 for my file servers or... ? :)
>
> It means you shouldn't use dd as a benchmark.

If you want to benchmark write speed, you should add
oflag=direct,dsync to the dd command line. For benchmarking read speed
you should specify iflag=direct. Or, even better, you can use xdd with
the flags -dio -processlock.

Bart.


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