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

To: "Tim Freeman" <tfreeman@xxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] RE: [Xen-users] cache for partition based VBD?
From: "Petersson, Mats" <mats.petersson@xxxxxxx>
Date: Tue, 29 Nov 2005 18:56:32 +0100
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Thread-topic: [Xen-users] cache for partition based VBD?
> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Tim Freeman
> Sent: 29 November 2005 17:12
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] cache for partition based VBD?
> 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? 

Let's first start with the facts:  don't know the answer to your

Now for some speculation: 
The block device cache would be efficient in the sense that it's useful
for every type of file-system, and it's not going to have to be clever
about what sort of accesses are frequent, not so frequent etc. This
gives benefit to the "cache at the device level".

On the other hand, a file-system level cache would also make sense.
Caching directory structurs or file-allocation-table [not necessarily in
the Microsoft sense] would obviously help a whole lot more than caching
some block from the middle of a file that isn't ever going to be read
again. This motivates having a targetted caching mechanism in the
file-system (VFS) layer. 

A combination would of course make even more sense - cache important
data structures in the File-system layer, and cache blocks that have
frequent/recent accesses in the block-device. 

> 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)

I would say that this is correct - the Dom0 disk driver would have a
cache in itself, and the DomU would also have a caching layer. But I'm
not SURE about this at all. 

If someone can make further comments, it would be great. 

> Thanks,
> Tim 
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