Re: [PATCH v11 04/16] scsi: core: Introduce a mechanism for reordering requests in the error handler

On 8/24/23 16:47, Bart Van Assche wrote:

On 8/23/23 16:22, Damien Le Moal wrote:

quoted

The sd driver does have zone append emulation using regular writes. The
emulation relies on zone write locking to avoid issues with adapters 
that do not
have strong ordering guarantees, but that could be adapted to be 
removed for UFS
devices with write ordering guarantees. This solution would greatly 
simplify
your series since zone append requests are not subject to zone write 
locking at
the block layer. So no changes would be needed at that layer.

However, that implies that your preferred use case (f2fs) must be 
adapted to use
zone append instead of regular writes. That in itself may be a bigger-ish
change, but in the long run, likely a better one I think as that would be
compatible with NVMe ZNS and also future UFS standards defining a zone 
append
command.

Hi Damien,

Thanks for the feedback. I agree that it would be great to have zone append
support in F2FS. However, I do not agree that switching from regular writes
to zone append in F2FS would remove the need for sorting SCSI commands 
by LBA in the SCSI error handler. Even if F2FS would submit zoned writes
then the following mechanisms could still cause reordering of the zoned
write after these have been translated into regular writes:
* The SCSI protocol allows SCSI devices, including UFS devices, to respond
with a unit attention or the SCSI BUSY status at any time. If multiple 
write commands are pending and some of the pending SCSI writes are not
executed because of a unit attention or because of another reason, this
causes command reordering.

Yes. But the important thing to remember is that with 'zone append' the 
resulting LBA will be returned on completion, they will _not_ be 
specified in the submission. So any command reordering doesn't affect 
the zone append commands as they heven't been written yet.

* Although the link between the UFS controller and the UFS device is pretty
reliable, there is a non-zero chance that a SCSI command is lost. If this
happens the SCSI timeout and error handlers are activated. This can cause
reordering of write commands.

Again, reordering is not an issue with zone append. With zone append you 
specify in which zone the command should land, and upon completion the 
LBA where the data is written will be returned.

So if there is an error the command has not been written, consequently 
there is no LBA to worry about, and you can reorder at will.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Ivo Totev, Andrew
Myers, Andrew McDonald, Martje Boudien Moerman