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Re: [Xen-users] cache for partition based VBD?

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] cache for partition based VBD?
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Wed, 30 Nov 2005 02:10:16 +0000
Cc: Tim Freeman <tfreeman@xxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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> For partition backed VBDs, is there still an effective cache in dom0?  The
> Linux buffer cache will affect file based VBDs, but does the partition
> backed VBD get around it?

AFAIK the blkback driver uses uncached interfaces to storage.  Caching only 
happens if you're using loop devices in dom0, because that indirects through 
the filesystem, which will being use the page cache.

You don't want caching in dom0, really: the domU will manage its own caches 
(and do so more effectively than dom0 could!) anyhow, so there's no point 
duplicating the effort.

> I think that the Linux architecture puts the buffer cache in front of all
> block device access, not at the VFS layer.  So does that answer the
> question?  The dom0 disk backend driver sits "above" the buffer cache? 
> (sorry for my crude understanding)

AFAIK, it sits *alongside* the page cache layer, as another client of the 
block system proper.


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