Re: [PATCH] block: use the request length for iov alignment

[Damien Le Moal]

On 2022/09/13 15:36, Keith Busch wrote:
> On Tue, Sep 13, 2022 at 03:20:23PM +0100, Damien Le Moal wrote:
>> On 2022/09/13 15:12, Keith Busch wrote:
>>> On Thu, Sep 08, 2022 at 09:45:26AM -0700, Keith Busch wrote:
>>>> From: Keith Busch <kbusch@kernel.org>
>>>>
>>>> An iov length needs to be aligned to the logical block size, which may
>>>> be larger than the memory alignment.
>>>
>>> [cc'ing some other interested folks]
>>>
>>> Any thoughts on this patch? It is fixing an observed IO error  when running
>>> virtio-blk with the default 512b logical block size backed by a drive 
>>> formatted
>>> with 4k logical block.
>>
>> The patch look OK to me, but having virtio expose a 512B LBA size for a 
>> backing
>> device that has 4K LBAs will break all IOs if caching is turned off (direct 
>> IOs
>> case), even if this patch is applied. No ?
> 
> Yeah, it's just the default. I don't think anyone would do that on purpose
> since it's so sub-optimal from a performance stand-point. As a follow up 
> patch,
> perhaps if the logical_block_size parameter is not provided, we should default
> to the backing device's block size?

That sound like a reasonable approach to me.

Note that I was not talking about the sub-optimal read-modify-write pattern for
IOs not aligned on the backend device sector boundaries, but rather thinking
about no IOs working at all if QEMU is started without caching (cache.direct=on
option)


-- 
Damien Le Moal
Western Digital Research