Re: [PATCH V2 0/1] ubd: add io_uring based userspace block driver

[Ming Lei]

On Tue, May 17, 2022 at 01:53:57PM +0800, Ming Lei wrote:
> Hello Guys,
> 
> ubd driver is one kernel driver for implementing generic userspace block
> device/driver, which delivers io request from ubd block device(/dev/ubdbN) into
> ubd server[1] which is the userspace part of ubd for communicating
> with ubd driver and handling specific io logic by its target module.
> 
> Another thing ubd driver handles is to copy data between user space buffer
> and request/bio's pages, or take zero copy if mm is ready for support it in
> future. ubd driver doesn't handle any IO logic of the specific driver, so
> it is small/simple, and all io logics are done by the target code in ubdserver.
> 
> The above two are main jobs done by ubd driver.
> 
> ubd driver can help to move IO logic into userspace, in which the
> development work is easier/more effective than doing in kernel, such as,
> ubd-loop takes < 200 lines of loop specific code to get basically same 
> function with kernel loop block driver, meantime the performance is
> still good. ubdsrv[1] provide built-in test for comparing both by running
> "make test T=loop".
> 
> Another example is high performance qcow2 support[2], which could be built with
> ubd framework more easily than doing it inside kernel.
> 
> Also there are more people who express interests on userspace block driver[3],
> Gabriel Krisman Bertazi proposes this topic in lsf/mm/ebpf 2022 and mentioned
> requirement from Google. Ziyang Zhang from Alibaba said they "plan to
> replace TCMU by UBD as a new choice" because UBD can get better throughput than
> TCMU even with single queue[4], meantime UBD is simple. Also there is userspace
> storage service for providing storage to containers.
> 
> It is io_uring based: io request is delivered to userspace via new added
> io_uring command which has been proved as very efficient for making nvme
> passthrough IO to get better IOPS than io_uring(READ/WRITE). Meantime one
> shared/mmap buffer is used for sharing io descriptor to userspace, the
> buffer is readonly for userspace, each IO just takes 24bytes so far.
> It is suggested to use io_uring in userspace(target part of ubd server)
> to handle IO request too. And it is still easy for ubdserver to support
> io handling by non-io_uring, and this work isn't done yet, but can be
> supported easily with help o eventfd.
> 
> This way is efficient since no extra io command copy is required, no sleep
> is needed in transferring io command to userspace. Meantime the communication
> protocol is simple and efficient, one single command of
> UBD_IO_COMMIT_AND_FETCH_REQ can handle both fetching io request desc and commit
> command result in one trip. IO handling is often batched after single
> io_uring_enter() returns, both IO requests from ubd server target and
> IO commands could be handled as a whole batch.
> 
> Remove RFC now because ubd driver codes gets lots of cleanup, enhancement and
> bug fixes since V1:
> 
> - cleanup uapi: remove ubd specific error code,  switch to linux error code,
> remove one command op, remove one field from cmd_desc
> 
> - add monitor mechanism to handle ubq_daemon being killed, ubdsrv[1]
>   includes builtin tests for covering heavy IO with deleting ubd / killing
>   ubq_daemon at the same time, and V2 pass all the two tests(make test T=generic),
>   and the abort/stop mechanism is simple
> 
> - fix MQ command buffer mmap bug, and now 'xfstetests -g auto' works well on
>   MQ ubd-loop devices(test/scratch)
> 
> - improve batching submission as suggested by Jens
> 
> - improve handling for starting device, replace random wait/poll with
> completion
> 
> - all kinds of cleanup, bug fix,..
> 
> And the patch by patch change since V1 can be found in the following
> tree:
> 
> https://github.com/ming1/linux/commits/my_for-5.18-ubd-devel_v2

BTW, a one-line fix[1] is added to above branch, which fixes performance
obviously on small BS(< 128k) test. If anyone run performance test,
please include this fix.

[1] https://github.com/ming1/linux/commit/fa91354b418e83953304a3efad4ee6ac40ea6110

Thanks,
Ming