Re: [Xen-users] RESOLVED: bad I/O performance with HP Smart Array RAID

[Thomas Aeby <aeby@xxxxxxxxxx>]

Timh B wrote:
> Yes, we are almost exclusively running SAS disk, so comparing to SATA
> disks may not be fair.
It's well probable that different experiences rely on the controller not
supporting SATA well. We usually use SATA disks in preference just in
order to have another option for a possible disaster recovery scenario.
Normal SATA vs SAS performance is normally not an issue for us, the
100-150MB/s you get normally out of a RAID 10 4-drive SATA array has always
been sufficient - but of course not the HP e200i 10-15MB/s.

> Even though sub-30mb writes is really low, perhaps
> the cciss driver is not good enough or compatible with xen-kernels?
As far as I see (look at the thread I pointed you to) the people
complaining about horribly poor performance are running a wide range of
different kernel versions, most of them non-Xen. So, I assume it is not
a Xen issue. Also, I've seen quite a few complaints of Windows users, 
so the issue does not seem to be a Linux driver issue, either. It's most
probably an issue between the controller and the drives - that might be
a reason why people are complaining that enabling BBWC does not help at
all (well, a few megs of cache is filled in milliseconds), while enabling
the drives' write cache seems to have solved the issue for many users.

> What were the exact measurements you used?
Oh, just the silly old "dd" to a large file and to an LVM volume - the
danger with "dd" is to get higher throughput figures because of caching
effects, so it is ok for determining max figures. But of course I did
test after getting complaints about the performance and after seeing
myself a DRBD mirror syncing with only about 10MB/s, either (after
having suspected someone had plugged the server into a FastEthernet
port I was quite surprised to see that this was not a network issue at 
all).
Best regards,
Tom
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Thomas Aeby, Kirchweg 52, 1735 Giffers, Switzerland
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