Re: [Xen-devel] xl block-attach vs block-detach

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 2 Mar 2012, Jan Beulich wrote:
> >>> On 02.03.12 at 14:49, Stefano Stabellini 
> >>> <stefano.stabellini@xxxxxxxxxxxxx>
> wrote:
> > On Fri, 2 Mar 2012, Ian Campbell wrote:
> >> > And if using blkback, why not (as in xend) via loop devices?
> >> 
> >> Support for block device script= in xl/libxl is on the 4.2 blocker list,
> >> this feature would re-enable the loop+blkback case -- I think this would
> >> be a better option than blktap* or qdisk once it becomes available.
> > 
> > Of course we need to make sure that blkback with loop devices performs
> > well before doing that, and it was certainly not the case in my last tests.
> 
> No - no policy should be involved here: If file:/ is specified, one should
> get blkback alone (no tap, qdisk, or what not. If another protocol was
> specified, that one should be used. Only if guessing is needed, some
> sort of heuristic (into which performance considerations may play) will
> (naturally) be required.

Let's suppose we find out that blkback+loop is slower than qdisk and
that block-attach/detach work well with qdisk by the time of the next
release.

What would be the rationale behind using blkback+loop for "file:"?
Backward compatibility?
Do you think it might break something for users if we change the backend
from xend to xl?

On the other hand do you think that using qdisk with the new disk syntax
introduced with xl is reasonable because users are not supposed to make
any assumptions there?

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