cache for partition based VBD?

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cache for partition based VBD?

Tim Freeman
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?

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)

Thanks,
Tim


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Re: cache for partition based VBD?

John Madden-4
> 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?

Filesystem caching shouldn't happen in dom0 for the domU's and I doubt you'd want
it to.  Caching *will* happen at the domU level and that's where you want it --
close to your applications.

Besides, if you can run a dom0 in like 128MB of memory to devote as much as
possible to your domU's, how much caching could you do?

John



--
John Madden
UNIX Systems Engineer
Ivy Tech Community College of Indiana
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Re: cache for partition based VBD?

Mark Williamson
In reply to this post by Tim Freeman
> 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.

Cheers,
Mark

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