Re: [Xen-devel] poor domU VBD performance.

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> What I was getting at was that the backend  will split requests
> up and issue each physical segment as a separate bio  (at least in
> the 2.0.5 tree I have in front of me). And that none of these
> physical segments was more that 1 page.
>
> So the request merging in the back end OS is important, no?
Ah, this reminds me I have one more question for Jens.

Since all the bio's that I queue up in a single invocation of 
dispatch_rw_block_io() will actually be adjacent to each other (because 
they're all from the same scatter-gather list) can I actually do 
something like (very roughly):
bio = bio_alloc(GFP_KERNEL, nr_psegs);
for ( i = 0; i < nr_psegs; i++ )
   bio_add_page(bio, blah...);
submit_bio(operation, bio);

Each of the biovecs that I queue may not be a full page in size (but 
won't straddle a page boundary of course).
This would avoid the bio's having to be merged again later.

 -- Keir


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