RE: [Xen-users] Block device caching causes out of memory in dom0

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> We run our dom0's with 128MB of RAM using xen 2.0.6, kernel 
> 2.6.11 for dom0 and domUs, and typically have ~10 domUs per 
> server.  All the domU filingsystems are loopback files served 
> from dom0 using the file method.

What are you using to back the loop devices? ext3 on local disk?

> Is there any way of stopping xen caching the loop devices?  I 
> thought about mapping a /dev/raw/rawX device onto /dev/loopX 
> (which works) but the /dev/raw/rawX devices are character 
> devices not block devices so I'm not sure how Xen would take to that.
> 
> Can Xen use the new O_DIRECT feature directly when it opens 
> the block device?  I imagine its doing it from kernel space 
> though so things may be different there.

I'm afraid this is a Linux loopback driver issue. It doesn't provide
back pressure if memory starts getting very dirty. Its actually quite
hard for it to do as the memory management system can make it hard for
it to tell.

> Would this be less of a problem if we switched over to using 
> partitions (lvm say)?  We are slightly reluctant to do this 
> due to previous bad experiences with lvm though!

LVM avoids the problem. We're looking at our own alternatives to the
loop driver.

Ian


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