Re: [RFC PATCH] ubd: add io_uring based userspace block driver

[Martin Raiber]

On 09.05.2022 11:23 Ming Lei wrote:
> This is the driver part of userspace block driver(ubd driver), the other
> part is userspace daemon part(ubdsrv)[1].
>
> The two parts communicate by io_uring's IORING_OP_URING_CMD with one
> shared cmd buffer for storing io command, and the buffer is read only for
> ubdsrv, each io command is indexed by io request tag directly, and
> is written by ubd driver.
>
> For example, when one READ io request is submitted to ubd block driver, ubd
> driver stores the io command into cmd buffer first, then completes one
> IORING_OP_URING_CMD for notifying ubdsrv, and the URING_CMD is issued to
> ubd driver beforehand by ubdsrv for getting notification of any new io request,
> and each URING_CMD is associated with one io request by tag.
>
> After ubdsrv gets the io command, it translates and handles the ubd io
> request, such as, for the ubd-loop target, ubdsrv translates the request
> into same request on another file or disk, like the kernel loop block
> driver. In ubdsrv's implementation, the io is still handled by io_uring,
> and share same ring with IORING_OP_URING_CMD command. When the target io
> request is done, the same IORING_OP_URING_CMD is issued to ubd driver for
> both committing io request result and getting future notification of new
> io request.
>
> Another thing done by ubd driver is to copy data between kernel io
> request and ubdsrv's io buffer:
>
> 1) before ubsrv handles WRITE request, copy the request's data into
> ubdsrv's userspace io buffer, so that ubdsrv can handle the write
> request
>
> 2) after ubsrv handles READ request, copy ubdsrv's userspace io buffer
> into this READ request, then ubd driver can complete the READ request
>
> Zero copy may be switched if mm is ready to support it.
>
> ubd driver doesn't handle any logic of the specific user space driver,
> so it should be small/simple enough.
>
> [1] ubdsrv
> https://github.com/ming1/ubdsrv/commits/devel

Great! It would be interesting to do some tests on how much faster (IOPS) this is than doing this via fuse: https://github.com/uroni/fuseuring