Re: [PATCH] nbd/server: Advertise MULTI_CONN for shared writable exports

[Vladimir Sementsov-Ogievskiy]

27.08.2021 18:09, Eric Blake wrote:
> According to the NBD spec, a server advertising
> NBD_FLAG_CAN_MULTI_CONN promises that multiple client connections will
> not see any cache inconsistencies: when properly separated by a single
> flush, actions performed by one client will be visible to another
> client, regardless of which client did the flush.  We satisfy these
> conditions in qemu because our block layer serializes any overlapping
> operations (see bdrv_find_conflicting_request and friends)

Not any. We serialize only write operations not aligned to request_alignment of 
bs (see bdrv_make_request_serialising() call in bdrv_co_pwritev_part). So, 
actually most of overlapping operations remain overlapping. And that's correct: 
it's not a Qemu work to resolve overlapping requests. We resolve them only when 
we are responsible for appearing of intersection: when we align requests up.

--
Best regards,
Vladimir