Re: [PATCH 00/26] Zone write plugging

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On 2/2/24 16:30, Damien Le Moal wrote:
> The patch series introduces zone write plugging (ZWP) as the new
> mechanism to control the ordering of writes to zoned block devices.
> ZWP replaces zone write locking (ZWL) which is implemented only by
> mq-deadline today. ZWP also allows emulating zone append operations
> using regular writes for zoned devices that do not natively support this
> operation (e.g. SMR HDDs). This patch series removes the scsi disk
> driver and device mapper zone append emulation to use ZWP emulation.
> Unlike ZWL which operates on requests, ZWP operates on BIOs. A zone
> write plug is simply a BIO list that is atomically manipulated using a
> spinlock and a kblockd submission work. A write BIO to a zone is
> "plugged" to delay its execution if a write BIO for the same zone was
> already issued, that is, if a write request for the same zone is being
> executed. The next plugged BIO is unplugged and issued once the write
> request completes.
> This mechanism allows to:
>  - Untangle zone write ordering from the block IO schedulers. This
>    allows removing the restriction on using only mq-deadline for zoned
>    block devices. Any block IO scheduler, including "none" can be used.
>  - Zone write plugging operates on BIOs instead of requests. Plugged
>    BIOs waiting for execution thus do not hold scheduling tags and thus
>    do not prevent other BIOs from being submitted to the device (reads
>    or writes to other zones). Depending on the workload, this can
>    significantly improve the device use and the performance.
>  - Both blk-mq (request) based zoned devices and BIO-based devices (e.g.
>    device mapper) can use ZWP. It is mandatory for the
>    former but optional for the latter: BIO-based driver can use zone
>    write plugging to implement write ordering guarantees, or the drivers
>    can implement their own if needed.
>  - The code is less invasive in the block layer and in device drivers.
>    ZWP implementation is mostly limited to blk-zoned.c, with some small
>    changes in blk-mq.c, blk-merge.c and bio.c.
> Performance evaluation results are shown below.
> The series is organized as follows:

I forgot to mention that the patches are against Jens block/for-next branch with
the addition of Christoph's "clean up blk_mq_submit_bio" patches [1] and my
patch "null_blk: Always split BIOs to respect queue limits" [2].


Damien Le Moal
Western Digital Research

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