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Re: [Qemu-devel] [PATCH 1/1] nbd: increase maximum size of the PWRITE_ZERO request

[resending with correct NBD list address]

On 02/08/2018 07:23 AM, Edgar Kaziakhmedov wrote:
Upstream NBD protocol implementation supports an efficient zero out
mechanism over the wire, along with the ability to check whether a
client allows using a hole.

Accordingly, since PWRITE_ZERO doesn't involve any payload on the wire,
increase a maximum size of the PWRITE_ZERO request up to 1Gb (aligned).
Moreover, such change will decrease the number of PWRITE_ZERO NBD commands
in comparison with the current 32M limit. The benefits of
the larger constraint can be examined in a block mirroring over NBD.

We've got a potential problem. Unless you have out-of-band communication of the maximum NBD_CMD_WRITE_ZEROES sizing (or if the NBD protocol is enhanced to advertise that as an additional piece of block size information during NBD_OPT_GO), then a client CANNOT assume that the server will accept a request this large. We MIGHT get lucky if all existing servers that accept WRITE_ZEROES requests either act on large requests or reply with EINVAL but do not outright drop the connection (which is different from servers that DO outright drop the connection for an NBD_CMD_WRITE larger than 32M). But I don't know if that's how all servers behave, so sending a too-large WRITE_ZEROES request may have the unintended consequence of killing the connection.

I'm adding the NBD list; perhaps before accepting this into qemu, I should revive my earlier attempt at codifying an NBD_OPT_GO info advertisement for maximum trim/zero sizing, which would let clients have a guarantee that their choice of sizing won't cause unexpected failures.

Signed-off-by: Edgar Kaziakhmedov <edgar.kaziakhmedov@virtuozzo.com>
  block/nbd.c         | 2 +-
  include/block/nbd.h | 3 +++
  2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/block/nbd.c b/block/nbd.c
index 94220f6d14..3641d9244e 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -477,7 +477,7 @@ static void nbd_refresh_limits(BlockDriverState *bs, Error **errp)
      uint32_t max = MIN_NON_ZERO(NBD_MAX_BUFFER_SIZE, s->info.max_block);

      bs->bl.max_pdiscard = max;
-    bs->bl.max_pwrite_zeroes = max;
+    bs->bl.max_pwrite_zeroes = NBD_MAX_PWRITE_ZERO_SIZE;

max_discard should get the same treatment, whatever we decide (yes, it is feasible that a server may have different limits for NBD_CMD_TRIM than for NBD_CMD_WRITE_ZEROES, but in general, it's probably easier if both commands share theh same limit as both have very similar implementations).

      bs->bl.max_transfer = max;

      if (s->info.opt_block &&
diff --git a/include/block/nbd.h b/include/block/nbd.h
index ee74ec391a..e2f18e2332 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -182,6 +182,9 @@ enum {
  /* Maximum size of a single READ/WRITE data buffer */
  #define NBD_MAX_BUFFER_SIZE (32 * 1024 * 1024)

+/* Maximum size of a single PWRITE_ZERO request 1Gb */
+#define NBD_MAX_PWRITE_ZERO_SIZE (1024 * 1024 * 1024)
  /* Maximum size of an export name. The NBD spec requires 256 and
   * suggests that servers support up to 4096, but we stick to only the
   * required size so that we can stack-allocate the names, and because

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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