[PATCH] doc/proto: drop max-block restriction for data-less commands
max-block restriction make sense for NBD_CMD_WRITE and NBD_CMD_READ to
avoid big data transfer and long io operations at server side.
NBD_CMD_WRITE_ZEROES still may be realized on server through direct
writing zeroes, which may lead to long operation and huge allocation
and should be restricted by max-block.
Same for NBD_CMD_CACHE: long operation / big allocation.
Still, NBD_CMD_TRIM, NBD_CMD_BLOCK_STATUS and NBD_CMD_WRITE_ZEROES with
NBD_CMD_FLAG_FAST_ZERO set are over-restricted by it. So, for better
performance, drop these restrictions.
Note, that Qemu nbd server already works accordingly to this patch: it
doesn't check the restriction for NBD_CMD_TRIM, NBD_CMD_BLOCK_STATUS
and NBD_CMD_WRITE_ZEROES.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
RFC question: hmm, Qemu nbd doesn't check restriction for WRITE_ZEROES
at all, even without NBD_CMD_FLAG_FAST_ZERO flag.
So, we probably could go further, and allow big WRITE_ZEROES without
this flag..
Or may be change s/MUST NOT/SHOULD NOT/ for this case..
doc/proto.md | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/doc/proto.md b/doc/proto.md
index fc7baf6..4b067f5 100644
--- a/doc/proto.md
+++ b/doc/proto.md
@@ -815,9 +815,12 @@ Where a transmission request can have a nonzero *offset* and/or
the client MUST ensure that *offset* and *length* are integer
multiples of any advertised minimum block size, and SHOULD use integer
multiples of any advertised preferred block size where possible. For
-those requests, the client MUST NOT use a *length* larger than any
-advertised maximum block size or which, when added to *offset*, would
-exceed the export size. The server SHOULD report an `NBD_EINVAL` error if
+those requests, the client MUST NOT use a *length* which, when added to
+*offset*, would exceed the export size. Also for NBD_CMD_READ,
+NBD_CMD_WRITE, NBD_CMD_CACHE and NBD_CMD_WRITE_ZEROES (except for
+when NBD_CMD_FLAG_FAST_ZERO is set), the client MUST NOT use a *length*
+larger than any advertised maximum block size.
+The server SHOULD report an `NBD_EINVAL` error if
the client's request is not aligned to advertised minimum block size
boundaries, or is larger than the advertised maximum block size.
Notwithstanding any maximum block size advertised, either the server
--
2.21.0
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