Re: [Qemu-block] [Qemu-devel] qemu NBD client behaviour when device size is not a multiple of 512

[Eric Blake]

On 07/31/2018 01:25 PM, Eric Blake wrote:
> On 07/31/2018 01:00 PM, Richard W.M. Jones wrote:
> > Hi Eric.  Is this a bug?

> Note that with qemu as server:
>
> $ truncate --size=1023 file
> $ qemu-nbd -f raw -x '' file
>
> the client now sees:
>
> address@hidden:nbd_opt_go_info_block_size Block sizes are 
> 0x200, 0x1000, 0x2000000
> ...
> address@hidden:nbd_receive_negotiate_size_flags Size is 1024, 
> export flags 0x1ed
>
> where qemu as server is 0-padding the source file in all read requests; 
> and that probably explains why I haven't noticed the problem with 
> unaligned images in the client before (since qemu as NBD server doesn't 
> advertise unaligned images).  What's more, a write request to the tail 
> of the partial sector (when not using a read-only connection) succeeds, 
> and resizes the file in the process (at least, on protocols that support 
> live resizing); but I wouldn't count on it behaving sanely for all 
> protocols.

What's more, with qemu as server, then 'qemu-img map' as client dies 
because the server is sending incorrect data according to protocol.

> At any rate, you've given me plenty to work with for fixing things, 
> although it probably won't make it into 3.0-rc3 and I don't know if 
> we'll have an -rc4.

Not a regression introduce in 3.0 (it exists in 2.12), so not serious 
enough of a show-stopper to cause an -rc4 release.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org