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

["Daniel P. Berrange" <berrange@...696...>]

On Wed, Feb 17, 2016 at 01:58:47PM -0700, Eric Blake wrote:
> On 02/17/2016 11:10 AM, Denis V. Lunev wrote:
> > This patch proposes a new command to reduce the amount of data passed
> > through the wire when it is known that the data is all zeroes. This
> > functionality is generally useful for mirroring or backup operations.
> > 
> > Currently available NBD_CMD_TRIM command can not be used as the
> > specification explicitely says that "a client MUST NOT make any
> 
> s/explicitely/explicitly/
> 
> > assumptions about the contents of the export affected by this
> > [NBD_CMD_TRIM] command, until overwriting it again with `NBD_CMD_WRITE`"
> > 
> > Particular use case could be the following:
> > 
> > QEMU project uses own implementation of NBD server to transfer data
> > in between different instances of QEMU. Typically we tranfer VM virtual
> 
> s/tranfer/transfer/
> 
> > disks over this channel. VM virtual disks are sparse and thus the
> > efficiency of backup and mirroring operations could be improved a lot.
> > 
> > Signed-off-by: Denis V. Lunev <den@...2317...>
> > ---
> >  doc/proto.md | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/doc/proto.md b/doc/proto.md
> > index 43065b7..c94751a 100644
> > --- a/doc/proto.md
> > +++ b/doc/proto.md
> > @@ -241,6 +241,8 @@ immediately after the global flags field in oldstyle negotiation:
> >    schedule I/O accesses as for a rotational medium
> >  - bit 5, `NBD_FLAG_SEND_TRIM`; should be set to 1 if the server supports
> >    `NBD_CMD_TRIM` commands
> > +- bit 6, `NBD_FLAG_SEND_WRITE_ZEROES`; should be set to 1 if the server
> > +  supports `NBD_CMD_WRITE_ZEROES` commands
> >  
> >  ##### Client flags
> >  
> > @@ -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))?

Stefan has suggested that we add a command to the NBD spec that
implements the SCSI Get LBA Status command. This lets clients
query the allocation bitmap for the device, which would serve
this purpose.

  https://lists.gnu.org/archive/html/qemu-devel/2016-02/msg03582.html

In that thread he talks about it being a way to serve up the dirty
bitmap for live backup scenario, but in regular usage it obviously
provides the normal allocation bitmap


Regards,
Daniel
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