Re: [Xen-users] Where is my CPU going?

["Luke S. Crawford" <lsc@xxxxxxxxx>]

On Wed, Jan 18, 2012 at 09:12:02AM -0800, Enzo Lombardi wrote:
> Hi all,
> I am trying to diagnose a lagging issue on one of my domains.
> Right now I have 3 DomU running + Dom0 and lagging is really bad. Even Dom0
> responsiveness is really low (sometimes it takes minutes to authenticate
> over SSH).

My experience?  90% of the time when something is that slow, it's disk
I/O that is the problem, not CPU.   (as someone else pointed out
DNS problems can cause big problems with network operations, which 
accounts for most of the other 10%, but it sounds like more than just 
network stuff is slow)  

Run 'top'  or 'sar' on the dom0 and look for IOwait.   If I'm right,
if you run top in one window while you try to ssh in from another, while
the SSH is being slow, the IOwait (I think in top, it says %wa)  will
be really high, like above 70%.

When that happens to me?  the first thing I look for is a guest that is
swapping a lot.  The next thing I look for is a failing disk.  Especially 
if you use consumer-grade disk rather than 'enterprise sata' when a
disk starts failing, it slows waay down.   The 'enterprise sata' stuff
tends to fail outright before that, which helps immensely.  



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