Re: [Qemu-devel] [RFC 1/1] nbd (specification): add NBD_CMD_WRITE_ZEROES command

[Roman Kagan]

On Wed, Feb 17, 2016 at 01:58:47PM -0700, Eric Blake wrote:
> On 02/17/2016 11:10 AM, Denis V. Lunev wrote:
> > @@ -446,6 +448,11 @@ The following request types exist:
> >      about the contents of the export affected by this command, until
> >      overwriting it again with `NBD_CMD_WRITE`.
> >  
> > +* `NBD_CMD_WRITE_ZEROES` (6)
> > +
> > +    A request to write zeroes. The command is functional equivalent of
> > +    the NBD_WRITE_COMMAND but without payload sent through the channel.
> 
> This lets us push holes during writes. Do we have the converse
> operation, that is, an easy way to query if a block of data will read as
> all zeroes, and therefore the client can bypass reading that portion of
> the disk (in other words, an equivalent to lseek(SEEK_HOLE/SEEK_DATA))?

The spec doesn't have anything like that.

OTOH, unlike the write case, where you have all the information and just
choose whether to send normal write or zero write, the extra round-trip
of a separate SEEK_HOLE/SEEK_DATA request may lead to actually degrading
the overall throughput.

Rather it may be a better idea to add something like sparse read where
the server would, instead of sending the full length of data in the
response payload, send a smarter variable-length package with a
scatter-gather list or a bitmap of used blocks in the beginning, and let
the client decode it and fill the gaps with zeros.

Roman.