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nbd: Implement NBD_CMD_WRITE_ZEROES on server

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Upstream NBD protocol recently added the ability to efficiently
write zeroes without having to send the zeroes over the wire,
along with a flag to control whether the client wants to allow
a hole.

Note that when it comes to requiring full allocation, vs.
permitting optimizations, the NBD spec intentionally picked a
different sense for the flag; the rules in qemu are:
MAY_UNMAP == 0: must write zeroes
MAY_UNMAP == 1: may use holes if reads will see zeroes

while in NBD, the rules are:
FLAG_NO_HOLE == 1: must write zeroes
FLAG_NO_HOLE == 0: may use holes if reads will see zeroes

In all cases, the 'may use holes' scenario is optional (the
server need not use a hole, and must not use a hole if
subsequent reads would not see zeroes).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1476469998-28592-16-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Eric Blake 2016-10-14 13:33:17 -05:00 • committed by Paolo Bonzini
parent b6f5d3b573
commit 1f4d6d18ed
2 changed files with 46 additions and 4 deletions
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8
include/block/nbd.h
View file

@ -71,6 +71,7 @@ typedef struct NBDReply NBDReply;
#define NBD_FLAG_SEND_FUA (1 << 3) /* Send FUA (Force Unit Access) */
#define NBD_FLAG_ROTATIONAL (1 << 4) /* Use elevator algorithm - rotational media */
#define NBD_FLAG_SEND_TRIM (1 << 5) /* Send TRIM (discard) */
#define NBD_FLAG_SEND_WRITE_ZEROES (1 << 6) /* Send WRITE_ZEROES */
/* New-style handshake (global) flags, sent from server to client, and
control what will happen during handshake phase. */
@ -96,7 +97,8 @@ typedef struct NBDReply NBDReply;
#define NBD_REP_ERR_SHUTDOWN NBD_REP_ERR(7) /* Server shutting down */
/* Request flags, sent from client to server during transmission phase */
#define NBD_CMD_FLAG_FUA (1 << 0)
#define NBD_CMD_FLAG_FUA (1 << 0) /* 'force unit access' during write */
#define NBD_CMD_FLAG_NO_HOLE (1 << 1) /* don't punch hole on zero run */
/* Supported request types */
enum {
@ -104,7 +106,9 @@ enum {
NBD_CMD_WRITE = 1,
NBD_CMD_DISC = 2,
NBD_CMD_FLUSH = 3,
NBD_CMD_TRIM = 4
NBD_CMD_TRIM = 4,
/* 5 reserved for failed experiment NBD_CMD_CACHE */
NBD_CMD_WRITE_ZEROES = 6,
};
#define NBD_DEFAULT_PORT 10809