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Re: [Xen-devel] Invalidating buffer cache in dom0

Ian Pratt wrote:

Have you tried using raw devices? See "man raw". I've never used them,
but I'd be interested to know whether this works. There's an argument
that we should be using them by default, particularly for people using
using cluster filesystems -- caching stuff in dom0 could be very bad.

It is my impression that in recent 2.6 kernels raw devices and O_DIRECT are the same thing. Actually, I was forced to use O_DIRECT inside my domU for the self-checkpointing, as otherwise the buffer cache would inflate the working set and create additional downtime. O_DIRECT works really well, and minus a ton of bugs I can now self-checkpoint to disk as well as self-migrate via the network.

O_DIRECT works for writes. However, it seems it still tries to be a little too clever when reading from a blockdev.

This is not a huge prob for me, just an annoyance that I have to reboot all the time to flush the cache, but I guess that for a Xen system in general it would be nice to have consistency of block device contents between domUs and dom0.


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