Re: SSD and IO schedulers

[Heinz Diehl <htd@fancy-poultry.org>]

On 08.04.2009, Corrado Zoccolo wrote: 

> I found that elevator=deadline performs much better than noop for
> writes, and almost as well for reads
[....]

The DL elevator has slightly more throughput than cfq and anticipatory,
but is almost unusuable under load.

Running Theodore Ts'os "fsync-tester" while doing Linus' torture test 
"while : ; do time sh -c "dd if=/dev/zero of=bigfile bs=8M count=256 ; sync; rm bigfile"; done"
shows it clearly:

mount: /dev/sda4 on /home type xfs (rw,noatime,logbsize=256k,logbufs=2,nobarrier)
Kernel 2.6.29.1 (vanilla)

with cfq:

htd@liesel:~/!> ./fsync-tester
fsync time: 0.7640
fsync time: 0.6166
fsync time: 1.2830
fsync time: 0.4273
fsync time: 1.1693
fsync time: 1.7466
fsync time: 1.2477
fsync time: 1.9411
fsync time: 1.9636
fsync time: 1.9065
fsync time: 1.1561
fsync time: 1.8267
fsync time: 0.2431
fsync time: 0.2898
fsync time: 0.2394
fsync time: 0.4309
fsync time: 1.5699
fsync time: 0.3742
fsync time: 1.3194
fsync time: 1.9442
fsync time: 1.0899
fsync time: 1.9443
fsync time: 1.0062

with dl:

fsync time: 10.5853
fsync time: 10.3339
fsync time: 5.3374
fsync time: 6.5707
fsync time: 10.6095
fsync time: 4.1154
fsync time: 4.9604
fsync time: 10.5325
fsync time: 10.4543
fsync time: 10.4970
fsync time: 10.5570
fsync time: 5.2717
fsync time: 10.5619
fsync time: 5.3058
fsync time: 3.1019
fsync time: 5.1504
fsync time: 5.7564
fsync time: 10.5998
fsync time: 4.0895


Regards, Heinz.