Thanks for the feedback everyone, I totally overlook chunk_sectors and
it sounds like that's exactly what we want. I'll send another patch
where that becomes writable.
On Mon, Jun 15, 2020 at 7:40 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
>
> On Mon, Jun 15, 2020 at 05:56:33PM -0700, Harshad Shirwadkar wrote:
> > This feature allows the user to control the alignment at which request
> > queue is allowed to split bios. Google CloudSQL's 16k user space
> > application expects that direct io writes aligned at 16k boundary in
> > the user-space are not split by kernel at non-16k boundaries. More
> > details about this feature can be found in CloudSQL's Cloud Next 2018
> > presentation[1]. The underlying block device is capable of performing
> > 16k aligned writes atomically. Thus, this allows the user-space SQL
> > application to avoid double-writes (to protect against partial
> > failures) which are very costly provided that these writes are not
> > split at non-16k boundary by any underlying layers.
> >
> > We make use of Ext4's bigalloc feature to ensure that writes issued by
> > Ext4 are 16k aligned. But, 16K aligned data writes may get merged with
> > contiguous non-16k aligned Ext4 metadata writes. Such a write request
> > would be broken by the kernel only guaranteeing that the individually
> > split requests are physical block size aligned.
> >
> > We started observing a significant increase in 16k unaligned splits in
> > 5.4. Bisect points to commit 07173c3ec276cbb18dc0e0687d37d310e98a1480
> > ("block: enable multipage bvecs"). This patch enables multipage bvecs
> > resulting in multiple 16k aligned writes issued by the user-space to
> > be merged into one big IO at first. Later, __blk_queue_split() splits
> > these IOs while trying to align individual split IOs to be physical
> > block size.
> >
> > Newly added split_alignment parameter is the alignment at which
> > requeust queue is allowed to split IO request. By default this
> > alignment is turned off and current behavior is unchanged.
> >
>
> Such alignment can be reached via q->limits.chunk_sectors, and you
> just need to expose it via sysfs and make it writable.
>
> Thanks,
> Ming
>
