Re: [Qemu-devel] BDRV request fragmentation and vitio-blk write submission guarantees

[Stefan Hajnoczi]

On Thu, Jul 18, 2019 at 04:44:17PM +0300, Евгений Яковлев wrote:
> Hi everyone,
> 
> We're currently working on implementing a qemu BDRV format driver which we
> are using with virtio-blk devices.
> 
> I have a question concerning BDRV request fragmentation and virtio-blk write
> request submission which is not entirely clear to me by only reading virtio
> spec. Could you please consider the following case and give some additional
> guidance?
> 
> 1. Our BDRV format driver has a notion of max supported transfer size. So we
> implement BlockDriver::bdrv_refresh_limits where we fill out
> BlockLimits::max_transfer and opt_transfer fields.
> 
> 2. virtio-blk exposes max_transfer as a virtio_blk_config::opt_io_size
> field, which (according to spec 1.1) is a **suggested** maximum. We read
> "suggested" as "guest driver may still send requests that don't fit into
> opt_io_size and we should handle those"...
> 
> 3. ... and judging by code in block/io.c qemu block layer handles such
> requests by fragmenting them into several BDRV requests if request size is >
> max_transfer
> 
> 4. Guest will see request completion only after all fragments are handled.
> However each fragment submission path can call qemu_coroutine_yield and move
> on to submitting next request available in virtq before completely
> submitting the rest of the fragments. Which means the following situation is
> possible where BDRV sees 2 write requests in virtq, both of which are larger
> than max_transfer:
> 
> ||
> 
> |Blocks: |-------------------------------------> Write1: xxxxxxxx Write2:
> yyyyyyyy Write1Chunk1: xxxx Write2Chunk1: yyyy Write2Chunk2: yyyy
> Write1Chunk1: xxxx Blocks: |------------yyyyxxxx----------------->|
> 
> ||
> 
> |In above scenario guest virtio-blk driver decided to submit 2 intersecting
> write requests, both of which are larger than ||max_transfer, and then call
> hypervisor.|
> 
> |I understand that virtio-blk may handle requests out of order, so guest
> must not make any assumptions on relative order in which those requests will
> be handled.|
> 
> |However, can guest driver expect that whatever the submission order will
> be, the actual intersecting writes will be atomic?|
> 
> |In other words, will it be correct for conforming virtio-blk driver to
> expect only "|||xxxxxxxx" or "||||yyyyyyyy" but not anything else in
> between, after both requests are reported as completed?||
> 
> ||Because i think that is something that may happen in qemu right now, if i
> understood correctly. ||

Write requests are not atomic in general.  Specific storage technologies
support atomic writes via special commands with certain restrictions but
applications using this feature aren't portable.

Portable applications either don't submit intersecting write requests or
they do not depend on atomicity.

Out of curiousity I took a quick look at Linux device-mapper.  The same
issue applies in device-mapper when intersecting write requests cross
device-mapper targets.  I think Linux submits split bios in parallel and
without serialization.

Stefan

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