Re: [PATCH 0/2] Submit split bios in LBA order

[Khazhy Kumykov <khazhy@chromium.org>]

On Mon, Mar 20, 2023 at 10:28 AM Bart Van Assche <bvanassche@acm.org> wrote:
>
> On 3/17/23 23:29, Christoph Hellwig wrote:
> > On Fri, Mar 17, 2023 at 12:59:36PM -0700, Bart Van Assche wrote:
> >> For zoned storage it is essential that split bios are submitted in LBA order.
> >> This patch series realizes this by modifying __bio_split_to_limits() such that
> >> it submits the first bio fragment and returns the remainder instead of
> >> submitting the remainder and returning the first bio fragment. Please consider
> >> this patch series for the next merge window.
> >
> > Why are you sending large writes using REQ_OP_WRITE and not
> > using REQ_OP_ZONE_APPEND which side steps all these issues?
>
> Hi Christoph,
>
> How to achieve optimal performance with REQ_OP_ZONE_APPEND for SCSI
> devices? My understanding of how REQ_OP_ZONE_APPEND works for SCSI
> devices is as follows:
> * ATA devices cannot support this operation directly because there are
>    not enough bits in the ATA sense data to report where appended data
>    has been written.
> * T10 has not yet started with standardizing a zone append operation.
> * The code that emulates REQ_OP_ZONE_APPEND for SCSI devices (in
>    sd_zbc.c) serializes REQ_OP_ZONE_APPEND operations (QD=1).
> * To achieve optimal performance, QD > 1 is required.
I recall there were dragons lurking particularly with how we handle
requeues wherein just submitting in order was not sufficient to
guarantee IO is actually dispatched in order. (of note: when
requeueing a request, we splice it to the _end_ of the hctx dispatch
list, so if you get a requeue in the middle of a multi-segment IO, it
will get re-ordered. I recall this change went in specifically to
re-order requests in case there was a passthrough lurking to un-jam a
device.) Have you looked at this? Perhaps requeues are slowpath
anyways, so we could sort there? There may also be other requeue
weirdness with layered devices...

Khazhy