Re: [Qemu-devel] [PATCH v3 6/6] nbd/server: Advertise actual minimum block size

[Eric Blake]

On 3/28/19 11:27 PM, Eric Blake wrote:
> Both NBD_CMD_BLOCK_STATUS and structured NBD_CMD_READ will split their
> reply according to bdrv_block_status() boundaries. If the block device
> has a request_alignment smaller than 512, but we advertise a block
> alignment of 512 to the client, then this can result in the server
> reply violating client expectations by reporting a smaller region of
> the export than what the client is permitted to address (although this
> is less of an issue for qemu 4.0 clients, given recent client patches
> to overlook our non-compliance at EOF).  Since it's always better to
> be strict in what we send, it is worth advertising the actual minimum
> block limit rather than blindly rounding it up to 512.
> 

> Note that the iotests output changes - both pre- and post-patch, the
> server is reporting a mid-sector hole; but pre-patch, the client was
> then rounding that up to a sector boundary as a workaround, while
> post-patch the client doesn't have to round because it sees the
> server's smaller advertised block size.
> 
> Signed-off-by: Eric Blake <address@hidden>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
>  nbd/server.c               | 13 ++++++++-----
>  tests/qemu-iotests/241.out |  3 ++-
>  2 files changed, 10 insertions(+), 6 deletions(-)

I missed some iotest changes; 223 and 233 also need patches along the
lines of:

@@ -41,7 +41,7 @@
  export: 'n'
   size:  4194304
   flags: 0x4ef ( readonly flush fua trim zeroes df cache )
-  min block: 512
+  min block: 1


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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