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Re: [Qemu-devel] [PATCH 05/12] block: add logical block provisioning inf

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 05/12] block: add logical block provisioning information to BlockDriverInfo
Date: Fri, 13 Sep 2013 14:58:48 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130805 Thunderbird/17.0.8

Il 13/09/2013 14:22, Peter Lieven ha scritto:
> 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).

Good idea.

> 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?

32768 (16 MB) is the highest power-of-two amount that fits in 16 bits
assuming 512-byte sectors.  But you could also take a look at what the
Linux kernel does in drivers/scsi/sd.c.


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