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Re: [Xen-users] Dom0 on USB Stick




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).
Hi Simon,

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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