nbd: Advertise multi-conn for shared read-only connections

The NBD specification defines NBD_FLAG_CAN_MULTI_CONN, which can be
advertised when the server promises cache consistency between
simultaneous clients (basically, rules that determine what FUA and
flush from one client are able to guarantee for reads from another
client).  When we don't permit simultaneous clients (such as qemu-nbd
without -e), the bit makes no sense; and for writable images, we
probably have a lot more work before we can declare that actions from
one client are cache-consistent with actions from another.  But for
read-only images, where flush isn't changing any data, we might as
well advertise multi-conn support.  What's more, advertisement of the
bit makes it easier for clients to determine if 'qemu-nbd -e' was in
use, where a second connection will succeed rather than hang until the
first client goes away.

This patch affects qemu as server in advertising the bit.  We may want
to consider patches to qemu as client to attempt parallel connections
for higher throughput by spreading the load over those connections
when a server advertises multi-conn, but for now sticking to one
connection per nbd:// BDS is okay.

See also: https://bugzilla.redhat.com/1708300
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190815185024.7010-1-eblake@redhat.com>
[eblake: tweak blockdev-nbd.c to not request shared when writable,
fix iotest 233]
Reviewed-by: John Snow <jsnow@redhat.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/block/nbd.h b/include/block/nbd.h
index 7b36d672f0..991fd52a51 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -326,7 +326,7 @@ typedef struct NBDClient NBDClient;
 
 NBDExport *nbd_export_new(BlockDriverState *bs, uint64_t dev_offset,
                           uint64_t size, const char *name, const char *desc,
-                          const char *bitmap, uint16_t nbdflags,
+                          const char *bitmap, uint16_t nbdflags, bool shared,
                           void (*close)(NBDExport *), bool writethrough,
                           BlockBackend *on_eject_blk, Error **errp);
 void nbd_export_close(NBDExport *exp);