Re: [Qemu-devel] [PATCH 05/12] block: add logical block provisioning information to BlockDriverInfo

[Peter Lieven <pl@kamp.de>]

Am 13.09.2013 13:45, schrieb Paolo Bonzini:
> Il 13/09/2013 12:44, Peter Lieven ha scritto:
>> On 13.09.2013 12:34, Paolo Bonzini wrote:
>>> Il 13/09/2013 12:25, Peter Lieven ha scritto:
>>>> +    /* maximum number of sectors that can be discarded at once */
>>>> +    int max_discard;
>>>> +    /* maximum number of sectors that can zeroized at once */
>>>> +    int max_write_zeroes;
>>> These should not be needed outside the driver.
>>>
>>> If you want to make them private between block.c and block/iscsi.c, you
>>> can add them to BlockDriverState.
>> The question is, if the discard_zeroes or discard_write_zeroes is needed
>> outside the driver as well?
>>
>> I can put the max_* information in the block driver state. I also thought
>> to add alignment and granularity information even if they are currently
>> not yet used.
> Yeah, in fact bdrv_write_zeroes and bdrv_discard can be taught to split
> requests according to these parameters instead of introducing a new
> function bdrv_zeroize.  You don't need bdrv_zeroize I think; you can
> simply use bdrv_write_zeroes.  This is why I don't like this information
> in BlockDriverInfo.
bdrv_zeroize has one big advantage over a bdrv_write_zeroes over
the whole device: it checks the block status before it sends requests.
this can be a great performance benefit if a lot of blocks are already
unmapped. so i would like to keep it in, but simplifiy it (see below).

>
> On the contrary, discard_write_zeroes is useful to "generic" clients,
> and your qemu-img patch shows why.
>
> Discard_zeroes is somewhere in the middle.  You only use it in
> bdrv_get_block_status, but it is not something that should be hidden to
> users of the high-level block.c API.  So it is fine to leave it in
> BlockDriverInfo.
okay, i will leave them in and put

max_discard
discard_alignment
max_write_zeroes
write_zeroes_alignment

into the BlockDriverState. I would then like to call out to all
driver developers to set these values for their drivers
to good values.

For now I can factor out the request split logic out of
iscsi_co_discard, iscsi_co_write_zeroes and bdrv_sanitize
and put them in bdrv_co_discard and bdrv_co_write_zeroes.

I would like to leave the misalignment logic to a later patch.

What would you think are reasonable default values for
max_discard and max_write_zeroes?

Peter