Re: [Qemu-devel] [Nbd] [Qemu-block] [PATCH] doc: Propose NBD_FLAG_INIT_ZEROES extension

[Alex Bligh]

> On 6 Dec 2016, at 09:25, Kevin Wolf <address@hidden> wrote:
> 
> Am 06.12.2016 um 00:42 hat Eric Blake geschrieben:
>> While not directly related to NBD_CMD_WRITE_ZEROES, the qemu
>> team discovered that it is useful if a server can advertise
>> whether an export is in a known-all-zeroes state at the time
>> the client connects.
> 
> Does a server usually have the information to set this flag, other than
> querying the block status of all blocks at startup? 

The server may have other ways of knowing this, for instance
that it has just created the file (*), or that it stat'd the file
before opening it (not unlikely) and noticed it had 0 allocated
size. The latter I suspect would be trivial to implement in nbd-server

(*) = e.g. I had one application where nbd use the export path
to signify it wanted to open a temporary file, the path consisting
of a UUID and an encoded length. If the file was not present already
it created it with ftruncate(). That could trivially have used this.

> If so, the client could just query this by itself.

Well there's no currently mainlined extension to do that, but yes
it could. On the other hand I see no issue passing complete
zero status back to the client if it's so obvious from a stat().

-- 
Alex Bligh