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

[Kevin Wolf]

Am 23.09.2014 um 08:15 hat Peter Lieven geschrieben:
> On 22.09.2014 21:06, Paolo Bonzini wrote:
> >Il 22/09/2014 11:43, Peter Lieven ha scritto:
> >>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.
> >Understood.  But the right fix is to avoid that backend limits transpire
> >into guest ABI, not to catch the limits earlier.  So the right fix would
> >be to implement request splitting.
> 
> Since you proposed to add traces for this would you leave those in?
> And since iSCSI is the only user of this at the moment would you
> go for implementing this check in the iSCSI block layer?
> 
> As for the split logic would you think it is enough to build new qiov's
> out of the too big iov without copying the contents? This would work
> as long as a single iov inside the qiov is not bigger the max_transfer_length.
> Otherwise I would need to allocate temporary buffers and copy around.

You can split single iovs, too. There are functions that make this very
easy, they copy a sub-qiov with a byte granularity offset and length
(qemu_iovec_concat and friends). qcow2 uses the same to split requests
at (fragmented) cluster boundaries.

Kevin