Re: [Qemu-devel] [PATCH 06/12] nbd: support NBD_CMD_TRIM in the server

[Paolo Bonzini]

On 09/14/2011 05:44 PM, Christoph Hellwig wrote:
> > Map it to bdrv_discard.  The server can now expose NBD_FLAG_SEND_TRIM.
>
> Note that discard support without a way to communicate the alignment/size
> requirements,

Yep, especially because alignment can be as small as 512 for sparse raw, 
and as high as 65536 for qcow2...

> and without the discard_zeroes_data flag is pretty much
> useless.

... but right now in QEMU it is most useful with qcow2, so 
!discard_zeroes_data is pretty much the best you can do.

In general, QEMU's block layer does not really do much to pass 
information on discard features, which is why I didn't think much about 
these bits.  I'm interested in making NBD as featureful as the QEMU 
block layer, but beyond that not much. :)

Still you're obviously right, I'll talk to upstream.

Paolo