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


From: Denis V. Lunev
Subject: Re: [Qemu-devel] [RFC 1/1] nbd (specification): add NBD_CMD_WRITE_ZEROES command
Date: Thu, 18 Feb 2016 17:05:02 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1

On 02/18/2016 03:14 PM, Daniel P. Berrange wrote:
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 <address@hidden>
---
  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
But in this case we should allow to query the information for
more than one block at once and also we will have to make
an agreement in between the client and server about
the granularity of the request or specify the granularity
along as the range in the call.

Den



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