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

[Peter Lieven <pl@kamp.de>]

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