Comment 423 for bug 500069

(In reply to comment #255)
> Matt - I don't think this test case works for you as much because you have
> such
> a fast disk array. I imagine that you can write 10GB pretty quickly with an
> 8-drive array. Try increasing the 10GB to 100GB and increasing the sleep to
> 20-30 seconds so that you get more data waiting to be flushed to disk.
>

Setting dirty_background_ratio=1 and dirty_ratio=2 had a HUGE effect on my system.

$ dd if=/dev/zero of=/var/tmp/bigfile bs=1M count=100000 conv=fdatasync & sleep 30 ; time dd if=/dev/zero of=/var/tmp/smallfile bs=4k count=1 conv=fdatasync
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 6.96642 s, 0.6 kB/s

real 0m8.590s
user 0m0.000s
sys 0m0.004s

100000+0 records in
100000+0 records out
104857600000 bytes (105 GB) copied, 1354.9 s, 77.4 MB/s

# echo 1 > dirty_background_ratio ; echo 2 > dirty_ratio

$ dd if=/dev/zero of=/var/tmp/bigfile bs=1M count=100000 conv=fdatasync & sleep 30 ; time dd if=/dev/zero of=/var/tmp/smallfile bs=4k count=1 conv=fdatasync
[1] 22718
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 0.72366 s, 5.7 kB/s

real 0m0.725s
user 0m0.000s
sys 0m0.001s

100000+0 records in
100000+0 records out
104857600000 bytes (105 GB) copied, 359.02 s, 292 MB/s