[Bug 10846] Slow write on LSISAS1068E (SAS6/iR) on kernel >= 2.6.22

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http://bugzilla.kernel.org/show_bug.cgi?id=10846





------- Comment #3 from dom.lalot@gmail.com  2008-06-03 00:00 -------
James and Roland

Sure there's write cache enabled and your remark make sense. That's the first
thing I noticed when changing kernel.
We went from 2.8.18
SCSI device sda: drive cache: write through
SCSI device sda: 285155328 512-byte hdwr sectors (146000 MB)

testing with kernels 2.6.22 or 2.6.24, we noticed a change about the write
cache, just changing the kernel on the same hardware.
[  115.986031] sd 4:1:0:0: [sdb] Write cache: disabled, read cache: enabled,
doesn't support DPO or FUA
[  115.986494] sd 4:1:0:0: [sdb] 285155328 512-byte hardware sectors (146000
MB)

So we got a utility lsiutil to change the settings in the firmware about write
cache, and we saw then write cache: enabled, but the speed stayed slow.
We also took a driver from lsi

About your remarks, I tested again using dd with a larger file (when untaring
the kernel there's also a difference but it is less basic than dd)
The way dd reports its speed  may be not very accurate I agree, but it does not
change from a kernel point of view.

Linux debian-test.pr.univmed.fr 2.6.21.7
debian-test:~# dd if=/dev/zero of=/root/test.cdrom  bs=10k count=100000
100000+0 enregistrements lus
100000+0 enregistrements écrits
1024000000 octets (1,0 GB) copiés, 5,39271 seconde, 190 MB/s
debian-test:~# dd if=/dev/zero of=/root/test.cdrom  bs=10k count=100000
100000+0 enregistrements lus
100000+0 enregistrements écrits
1024000000 octets (1,0 GB) copiés, 5,45364 seconde, 188 MB/s
debian-test:~# dd if=/dev/zero of=/root/test.cdrom  bs=10k count=200000
200000+0 enregistrements lus
200000+0 enregistrements écrits
2048000000 octets (2,0 GB) copiés, 23,0492 seconde, 88,9 MB/s
debian-test:~# dd if=/dev/zero of=/root/test.cdrom  bs=10k count=200000
200000+0 enregistrements lus
200000+0 enregistrements écrits
2048000000 octets (2,0 GB) copiés, 22,5306 seconde, 90,9 MB/s

I reboot and change kernel:
debian-test:~# uname -a
Linux debian-test.pr.univmed.fr 2.6.22.19 #1 SMP Fri May 30 19:53:56 CEST 2008
i686 GNU/Linux
debian-test:~# dd if=/dev/zero of=/root/test.cdrom  bs=10k count=100000
100000+0 enregistrements lus
100000+0 enregistrements écrits
1024000000 octets (1,0 GB) copiés, 13,9614 seconde, 73,3 MB/s
debian-test:~# dd if=/dev/zero of=/root/test.cdrom  bs=10k count=100000
100000+0 enregistrements lus
100000+0 enregistrements écrits
1024000000 octets (1,0 GB) copiés, 13,9406 seconde, 73,5 MB/s
debian-test:~# dd if=/dev/zero of=/root/test.cdrom  bs=10k count=200000
200000+0 enregistrements lus
200000+0 enregistrements écrits
2048000000 octets (2,0 GB) copiés, 29,3472 seconde, 69,8 MB/s
debian-test:~# dd if=/dev/zero of=/root/test.cdrom  bs=10k count=200000
200000+0 enregistrements lus
200000+0 enregistrements écrits
2048000000 octets (2,0 GB) copiés, 29,1689 seconde, 70,2 MB/s
debian-test:~#

(I believe that, if I take a bigger file, we will get the same speed, the
difference is due to the first data going to write cache)

What can we see:
1. having a larger file make the write cache less efficient (normal)
2. It seems that the write caching is no more working from 2.6.22 on our
hardware (new blade servers from Dell m600). Even using firmware utilities
didn't improve the speed. LSI firmware does not activate write cache and their
BIOS has no setup for that. Switching from 2.6.18 to 2.6.22 makes the kernel 
no more doing write cache. Changing in the firmware activate something.. just
in dmesg, we see it: enabled again, but in fact there's no speed difference.

My subject should have  been: no more write caching


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