Re: Invalidating buffer cache in dom0

[Jacob Gorm Hansen <jacobg@diku.dk>]

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.

thanks,
Jacob