Very poor RAID write performance

  • Thread starter Thread starter Pauldoo
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Pauldoo

Hi,
We have HP xw9400 workstations featuring LSI MegaRAID SAS 8344ELP
cards with 4 drives. Read performance is fine, but writing is
terrible. We only get 7 Mb/sec for sequential writes to RAID5, and
around 11 Mb/sec to RAID0.

We have tried using both Windows XP 64-bit and Windows 2003 32-bit,
the write performance is the same on both.

We have executed our benchmark script on a couple of other machines in
the office to give ourselves confidence in our test. Our normal
desktop machines manage around 43 Mb/sec, and one of our Dell 2U
servers with a 4 drive RAID1+0 manages 100 Mb/sec. These numbers seem
about right.

I am suspicious that the RAID BIOS does not let us enable write back
caching and instead forces us to use write through. It forces us to
do this because we do not have the battery backup module for this
card. Despite us being forced to use write through, I'm still shocked
that the performance is so terrible.


Does anyone know what might be wrong with our setup?


HP xw9400 spec:
http://h10010.www1.hp.com/wwpc/us/en/sm/WF05a/12454-12454-296719-307907-296721-3211286.html

LSI MegaRAID SAS 8344ELP card:
http://www.lsi.com/storage_home/pro.../megaraid_sas/megaraid_sas_8344elp/index.html
 
Previously Pauldoo said:
Hi,
We have HP xw9400 workstations featuring LSI MegaRAID SAS 8344ELP
cards with 4 drives. Read performance is fine, but writing is
terrible. We only get 7 Mb/sec for sequential writes to RAID5, and
around 11 Mb/sec to RAID0.
Click to expand...
We have tried using both Windows XP 64-bit and Windows 2003 32-bit,
the write performance is the same on both.
Click to expand...
We have executed our benchmark script on a couple of other machines in
the office to give ourselves confidence in our test. Our normal
desktop machines manage around 43 Mb/sec, and one of our Dell 2U
servers with a 4 drive RAID1+0 manages 100 Mb/sec. These numbers seem
about right.
Click to expand...

Same or different controller?
I am suspicious that the RAID BIOS does not let us enable write back
caching and instead forces us to use write through. It forces us to
do this because we do not have the battery backup module for this
card. Despite us being forced to use write through, I'm still shocked
that the performance is so terrible.
Click to expand...

Well, if it does weite-through on sector level, the performance
is about right. However this type of implementation would
be either extremely stupid or dictated by marketing
to make you buy the battery module.
Does anyone know what might be wrong with our setup?
Click to expand...

Nothing, that I can see. The fast reading also means that
the interfaces of the disks are working. Maybe one possibility:
If one of the disk has slight head positioning problems, then it
could be slow on writes, but still fast on reads. The way to determine
that is to look at the individual disks's SMART data.

Other than that I would say this looks like yet another
badly implemented hardware RAID controller to me.

Arno
 
Same or different controller?
Click to expand...

Our desktop machines don't have RAID controllers and the 2U Dell has
some integrated Dell controller.

Well, if it does weite-through on sector level, the performance
is about right. However this type of implementation would
be either extremely stupid or dictated by marketing
to make you buy the battery module.


Nothing, that I can see. The fast reading also means that
the interfaces of the disks are working. Maybe one possibility:
If one of the disk has slight head positioning problems, then it
could be slow on writes, but still fast on reads. The way to determine
that is to look at the individual disks's SMART data.
Click to expand...

We have a number of identical xw9400 boxes and they are all the same.
So I doubt the disks are at fault.

Other than that I would say this looks like yet another
badly implemented hardware RAID controller to me.

Arno
Click to expand...


Thanks for your reply. It sounds like you believe it's a crud card
(marketing's fault or otherwise).
 
Our desktop machines don't have RAID controllers and the 2U Dell has
some integrated Dell controller.
Click to expand...
We have a number of identical xw9400 boxes and they are all the same.
So I doubt the disks are at fault.


Thanks for your reply. It sounds like you believe it's a crud card
(marketing's fault or otherwise).
Click to expand...

Seems to be the case. After a very bad experience with a pretty
expensive Adaptec SATA card, I have stayed away from hardware RAID.
Software RAID is equally fast, more configurable and you do not
neet a spare RAID controller. At least under Linux and that is what
my servers run.

Arno
 
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