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 <d...@openvz.org> > > --- > > 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 -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|
