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))? -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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