Re: [Qemu-block] [PATCH 3/6] null: Switch to byte-based read/write

[Eric Blake]

On 04/24/2018 12:19 PM, Kevin Wolf wrote:

>>>> +static void null_refresh_limits(BlockDriverState *bs, Error **errp)
>>>> +{
>>>> +    bs->bl.request_alignment = 1;
>>>> +}
>>>
>>> I would rather modify bdrv_refresh_limits() so that it defaults to 1 for
>>> drivers supporting either .bdrv_co_preadv or .bdrv_aio_preadv.
>>
>> Sure, I can do that (although then I may have to provide a
>> refresh_limits callback for each of the other drivers that I
>> specifically left at 512 alignment).
> 
> Do we know that the other drivers need 512, or do we expect than they
> can handle byte granularity, but we try to err on the safe side?

Done to err on the safe side and avoid the difficulty of a deep audit in
code I'm unfamiliar with; in fact, for vxhs, I wasn't even able to
compile test things, which means I certainly wasn't able to reproduce a
test to see if dropping the alignment would break anything under qemu-io.

> 
> If it's the latter (which would be my expectation), then it's probably
> better to set request_alignment = 512 with a comment in those drivers
> and hope that someone will lift the restriction in the future.

Indeed, leaving a good comment for a future reader is worth the effort.
I'll send a v2 later today.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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