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RE: [Xen-users] SAR in dom0 or domU

I would assume that would be domU territory, unless somebody can provide any other means to gather that information.


If your system is reliable and you haven’t had any wall clock issues in your domains, it *should* be accurate, however I cannot confirm this (I’m not even sure how I would).  There were various wall clock problems (read: time went backwards, fast clocks, etc) in domains in 3.0.x under certain conditions, I believe that these have been fixed in 3.1/3.2 xen releases (some patches for clock issues were submitted, and no recent bug reports I have found).


Also, independent wallclock might (this is tentative, I’m not sure if domU clocks are generated on timer interrupts or when the domU resumes, the former would be fine, the later would be bad) introduce it’s own odd skew with heavily loaded systems due to the irregular scheduling and updating of the domU clocks. Leaving the default behaviour for clock synch should be considered best practice in this case (and most cases for that matter).


So in short, for >3.0.x, with independent wallclock disabled (default behaviour), yes.

                      >3.0.x, with independent wallclock enabled, maybe.

                      3.0.x, likely not on smp, maybe not regardless.




From: ridleys@xxxxxxxxx [mailto:ridleys@xxxxxxxxx] On Behalf Of Edoardo Ceccarelli
Sent: April 13, 2008 2:37 PM
To: [STD]Ein
Cc: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] SAR in dom0 or domU


The abstraction that Xen gives should be enough to give reliable values in certain conditions, I was wondering about disk troughput for example:
using LVM the dom0 sar shouldn't even be able to produce values cause it cannot mount the partition
on the other hand the xenU domain should give pretty reliable values... has anybody tried to compare sar results on xen?
...about cpu's, I completely agree with you

On Sun, Apr 13, 2008 at 5:38 PM, [STD]Ein <ein@xxxxxxxxxxxxxx> wrote:

It depends on what information you are trying to obtain.


If you want information about real hardware (cpu usage, nic's in dom0 etc) installing in dom0 would be the best case.  Installing in domU can give you good info about individual domain specific memory, network, tty usage etc, but cpu percentages may be quite artificial, and you wouldn't be able to see real hardware bottlenecks without examining the other domains.


From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Edoardo Ceccarelli
Sent: April 11, 2008 8:39 PM
To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] SAR in dom0 or domU



I was wondering about sar under xen: is it more logical to install it on dom0 or in each domU?
about disk i/o it should be able to give disk throughputs even in a dom0, and maybe, also about cpu usage, memory, etc.

certanly installing sar in a domU will give those values, but will this be reliable?

what's your opinion about this?


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