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Re: [Xen-devel] [PATCH] Blktap: Userspace file-based image support. (RFC)

On 19/6/06 10:56 pm, "Anthony Liguori" <aliguori@xxxxxxxxxx> wrote:

Julian Chesterfield wrote:

On 19/6/06 8:15 pm, "Anthony Liguori" <aliguori@xxxxxxxxxx> wrote:

Couple general comments on the code:

Please don't introduce more (ab)uses of /proc.  Sure it's just for
debugging but there's no reason to not make that sysfs.

I'm not an expert here, but the nopage handlers that I've seen return
NOPAGE_SIGBUS instead of manually causing a SIGBUS on current.

I think it's better to use C99 initialization than GCC:

  owner:  ...,  =>  .owner = ...,

Some of the indenting is a bit off from Linux CodingStyle. Stuff like
if( => if ( and some random spaces after an (.

There's some code commented out with C++ comments too.

What's the significance of /**BLKTAP**/ and /**TAPEND**/?

I'm a little surprised to see these conversion tools too. Wouldn't it
be easier to just add some parameters to qemu-img?

Thanks for the comments anthony. When we initially played with qcow
images it was easier to knock-up our own frontend to the plugins for
converting between the different image types and testing features like
image sparseness.  We added an optimisation feature in the xen qcow
plugin which would allocate full extents for non backing file based
images as well as the asynchronous callback architecture to enable
request batching for AIO.

We could certainly adapt qemu-img to use these and other features. Not
sure what the best approach for keeping the toolsets in synch between
the 2 projects would be though.

It may be worth just bringing up the changes on qemu-devel. I know why you'd want to change the cluster size (it's a pain to work with clusters
< block size).  I saw another comment about making metadata more
coarse.  Can you clarify the reasons for that?

We've been thinking about an enhancement to the qcow driver to use smarter readahead on the request ring in order to speculatively limit the number of metadata writes where request batching is used. This is an advantage of having access to the full frontend request queue which enables the userspace agent to make smart decisions regarding caching and safe but minimal metadata writes.

(Not sure which comment you'd read, but hope this may answer it!)

- Julian

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