Hi Folks,
I've been doing some experimenting to see how far I can push some old
hardware into a virtualized environment - partially to see how much use
I can get out of the hardware, and partially to learn more about the
behavior of, and interactions between, software RAID, LVM, DRBD, and Xen.
What I'm finding is that it's really easy to get into a state where one
of my VMs is spending all of its time in i/o wait (95%+). Other times,
everything behaves fine.
So... I'm curious about where the bottlenecks are.
What I'm running:
- two machines, 4 disk drives each, two 1G ethernet ports (1 each to the
outside world, 1 each as a cross-connect)
- each machine runs Xen 3 on top of Debian Lenny (the basic install)
- very basic Dom0s - just running the hypervisor and i/o (including disk
management)
---- software RAID6 (md)
---- LVM
---- DRBD
---- heartbeat to provide some failure migration
- each Xen VM uses 2 DRBD volumes - one for root, one for swap
- one of the VMs has a third volume, used for backup copies of files
What I'd like to dig into:
- Dom0 plus one DomU running on each box
- only one of the DomUs is doing very much - and it's runnin about 90%
idle, the rest split between user cycles and wait cycles
- start a disk intensive job on the DomU (e.g., tar a bunch of files on
the root LV, put them on the backup LV)
- i/o WAIT goes through the roof
It's pretty clear that this configuration generates a lot of complicated
disk activity. Since DRBD is at the top of the disk stack, I figure
this list is a good place to ask the question:
Any suggestions on how to track down where the delays are creeping in,
what might be tunable, and any good references on these issues?
Thanks very much,
Miles Fidelman
--
In theory, there is no difference between theory and practice.
In<fnord> practice, there is. .... Yogi Berra
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