Re: [PATCH 2/2] block: Split and submit bios in LBA order

[Bart Van Assche <bvanassche@acm.org>]

On 3/23/23 15:53, Damien Le Moal wrote:
> On 3/24/23 01:27, Bart Van Assche wrote:
>> On 3/23/23 03:28, Damien Le Moal wrote:
>>> For the zone append emulation, the write locking is done by sd.c and the upper
>>> layer does not restrict to one append per zone. So we actually could envision a
>>> UFS version of the sd write locking calls that is optimized for the device
>>> capabilities and we can keep a common upper layer (which is preferable in my
>>> opinion).
>>
>> I see a blk_req_zone_write_trylock() call in
>> sd_zbc_prepare_zone_append() and a blk_req_zone_write_unlock() call in
>> sd_zbc_complete() for REQ_OP_ZONE_APPEND operations. Does this mean that
>> the sd_zbc.c code restricts the queue depth to one per zone for
>> REQ_OP_ZONE_APPEND operations?
> 
> Yes, since the append becomes a regular write and HBAs are often happy to
> reorder these commands, even for SMR, we need the locking.
> 
> But if I understand your use case correctly, given that UFS gives guarantees on
> the command dispatching order, you could probably relax this locking for zone
> append requests. But you cannot for regular writes as the locking is up in the
> block layer and needed to avoid block layer level reordering.

Hi Damien,

I don't think that we can achieve QD > 1 even if we would switch to
REQ_OP_ZONE_APPEND. A SCSI LLD is allowed to respond with 
SCSI_MLQUEUE_HOST_BUSY if the SCSI core asks it to queue a command. This 
may lead to reordering and hence may cause UNALIGNED WRITE COMMAND 
errors. We want to avoid these errors.

Thanks,

Bart.