On 13/12/2011 22:55, xenusers wrote:
On Tue, Dec 13, 2011 at 4:39 PM, Jonathan
Tripathy <jonnyt@xxxxxxxxxxx>
wrote:
On 13/12/2011 22:28, Simon Hobson wrote:
Jonathan Tripathy wrote:
I'm currently mulling over the idea of running our Dom0
on a USB flash disk. Currently, we have the Dom0 and
DomU running of the same disk array. I'd like to know
the list's opinion on doing this. It seems like a very
nice idea to split the single critical Dom0 and the tens
of DomUs.
I'd like to stop the DomUs from affecting the disk IOPS
available for the Dom0 as I'm sure you'll appreciate the
critical nature of the it.
I'm not sure what you'd gain. Once the system is running,
Dom0 shouldn't be doing much (if any) disk I/O - and if it
is, then doing it to a USB memory stick is likely to be
more of a bottleneck to the system as a whole than a few
deeks on the hard disk(s).
Sometimes during times when the DomUs are busy, xm commands
take a long time to execute. I always presumed that this was
due to disk IO. Could it be something else?
Thanks
Hi Jonathan,
I don't know what hardware you're running on, but I can
tell you that I'm running Xen on some clustered PowerEdge
R815s (48 cores, 64GB RAM). During initial setup and testing,
I hadn't pinned Dom0 to just specific cores, we were just
letting it run as-is while we were ironing out the pacemaker
configuration to cluster them. The Xen resource agent was
occasionally timing out (meaning an 'xm list --long <domu
name>' was taking more than 30 seconds to complete) and
restarting the DomUs. I was somewhat baffled, as these
servers were oviously very bored with a testing-only load of a
few guests. I could run the command in a loop, sleeping for a
second each time, and sure enough it was rather slow... very
frequently taking 10 to 15 seconds to complete when there were
two DomUs running, occasionally 45 seconds or more.
Similarly, 'xm info' would take ages to return every few
times it was run.
Long story short (too late?), I added the "dom0_max_vcpus=2
dom0_vcpus_pin" arguments to the grub commandline for Xen.
Configured all DomUs to stay off of those cores with
'cpus="2-47"' in their config files. No other changes made to
the setup at all, but now the commands return almost
instantly, every single time. Giving dom0 only two cores
completely resolved the issue for us.
Regards,
Mark
Hi Mark,
Thanks for sharing the above info.
Indeed, my issue could be related to CPU. I have found that the
issue isn't so bad on our newer servers (which have better CPUs in
them). Unfortunately, we don't have enough cores to be able to
dedicate an entire core to the Dom0.
I guess this isn't the worst issue in the world for us, as the delay
is acceptable on our newer servers. Now, on our older servers (which
are due to be replaced fairly soon), the times can be as long as 30
- 60 seconds which is clearly too long!
Cheers
Jonathan
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