Re: RAID-1 performance under 2.4 and 2.6

[Bill Davidsen <davidsen@tmr.com>]

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... ? :)
> 
Unfortunately this shows the same trend as kernel compile, small 
database operations, etc. If you are using a journaling filesystem on 
2.6 and not 2.4 be sure you have the filesystem mounted "noatime" or 
retest with a non-journaled f/s. If you are running LVM in the test all 
bets are off as there are alignment issues (see linux-raid archives) to 
consider.

But the trend has unfortunately been slower, and responses demanding you 
use another benchmark, saying that kernel compile is not a benchmark, 
suggesting use of postgress or oracle instead of MySQL, etc, are seen.

I wish it were not so, there seems to be more effort going to explaining 
results than improving them. That said, tuning the location of the f/s, 
the stride, chunk size, etc, can improve things, and there are patches 
available for test (linux-raid again) which will address some of this 
fairly soon.

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