[Xen-users] strange i/o hangs on debian lenny guests

[John Madden <jmadden@xxxxxxxxxxx>]

I haven't seen this on the list before but I'm wondering if anyone else 
has experienced this or has any ideas on what I'm finding to be very 
strange (and bad) i/o behavior.  I have soft hangs on four Debian lenny 
guests backed by LVM that I can trigger reliably with bonnie++ by 
running it and then firing off something in another session that isn't 
cached and thus has to go to disk (like running 'who' instead of 'w').
The second command will eventually come back but it takes about 90 
seconds.  Further testing showed that no i/o was actually going on 
during those 90 seconds.  So as the scenario goes, start up `vmstat 1` 
in a shell and then `bonnie++ -r 1024` in another.  Let that bonnie++ 
run for 15 seconds or so so you run out your buffer cache, then try to 
launch something else that has to read from disk.
The vmstat counters will drop to 0 on bi and bo and anything requiring 
disk activity will hang for exactly 90 seconds.  After 90 seconds, i/o 
picks up where it left off and everything will appear to be ok.
I've played with i/o schedulers and various kernel versions (even 
running etch's kernel) to no avail.  Any ideas?
This is on a CentOS 5.4 hypervisor, Debian 5 domU with 1GB memory, 2 
vcpu's, single LVM phy: disk.
Thanks,
  John




--
John Madden
Sr UNIX Systems Engineer
Ivy Tech Community College of Indiana
jmadden@xxxxxxxxxxx

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