Re: [Qemu-devel] [PATCH 4/4] block: avoid creating oversized writes in multiwrite_merge

[Peter Lieven]

On 19.09.2014 15:33, Paolo Bonzini wrote:
> Il 19/09/2014 00:56, Peter Lieven ha scritto:
> > > > So I think if we treat it just as a hint for multiwrite, we can avoid
> > > > writing code to split oversized requests.  They always worked so far, we
> > > > can certainly wait until we have a real bug fix.
> > I would not treat this as a hint. I would use it in cases where we definitely
> > know an absolute hard limit for I/O request size. Otherwise the value for
> > bs->bl.max_transfer_length should be 0.
> >
> > If there comes in an oversized request we fail it as early as possible
> That's the part that I'd rather not touch, at least not without doing
> request splitting.

This series aims not at touching default behaviour. The default for 
max_transfer_length
is 0 (no limit). max_transfer_length is a limit that MUST be satisfied 
otherwise the request
will fail. And Patch 2 aims at catching this fail earlier in the stack. 
Currently, we only
have a limit for iSCSI. Without Patch 2 it would fail after we have send the 
command to
the target. And without Patch 4 it may happen that multiwrite_merge traps the 
into the limit.

Maybe I should adjust the description of max_transfer_length?

Peter