[Report] requests are submitted to hardware in reverse order from nvme/virtio-blk queue_rqs()

[Damien Le Moal]

On 1/25/24 00:41, Keith Busch wrote:
> On Wed, Jan 24, 2024 at 07:59:54PM +0800, Ming Lei wrote:
>> Requests are added to plug list in reverse order, and both virtio-blk
>> and nvme retrieves request from plug list in order, so finally requests
>> are submitted to hardware in reverse order via nvme_queue_rqs() or
>> virtio_queue_rqs, see:
>>
>> 	io_uring       submit_bio  vdb      6302096     4096
>> 	io_uring       submit_bio  vdb     12235072     4096
>> 	io_uring       submit_bio  vdb      7682280     4096
>> 	io_uring       submit_bio  vdb     11912464     4096
>> 	io_uring virtio_queue_rqs  vdb     11912464     4096
>> 	io_uring virtio_queue_rqs  vdb      7682280     4096
>> 	io_uring virtio_queue_rqs  vdb     12235072     4096
>> 	io_uring virtio_queue_rqs  vdb      6302096     4096
>>
>>
>> May this reorder be one problem for virtio-blk and nvme-pci?
> 
> For nvme, it depends. Usually it's probably not a problem, though some
> pci ssd's have optimizations for sequential IO that might not work if
> these get reordered.

ZNS and zoned virtio-blk drives... Cannot use io_uring at the moment. But I do
not thing we reliably can anyway, unless the issuer is CPU/ring aware and always
issue writes to a zone using the same ring.

-- 
Damien Le Moal
Western Digital Research