On 1/4/19 4:08 PM, Richard W.M. Jones wrote: >>From the protocol document: > > "NBD_FLAG_CAN_MULTI_CONN: Indicates that the server operates > entirely without cache, or that the cache it uses is shared among > all connections to the given device. In particular, if this flag is > present, then the effects of NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA MUST > be visible across all connections when the server sends its reply to > that command to the client. In the absence of this flag, clients > SHOULD NOT multiplex their commands over more than one connection to > the export." > > This is implemented in nbdkit by a boolean function which plugins may > implement if they believe they satisfy the requirements above. It can > also be passed through or modified by filters. It defaults to false, > which is the safe default. > --- > docs/nbdkit-filter.pod | 4 ++++ > docs/nbdkit-plugin.pod | 20 ++++++++++++++++++++ > docs/nbdkit-protocol.pod | 4 ++++ > include/nbdkit-filter.h | 3 +++ > include/nbdkit-plugin.h | 2 ++ > server/internal.h | 1 + > server/protocol.h | 1 + > server/connections.c | 9 +++++++++ > server/filters.c | 24 ++++++++++++++++++++++++ > server/plugins.c | 17 +++++++++++++++++ > tests/test-layers-filter.c | 10 ++++++++++ > tests/test-layers-plugin.c | 8 ++++++++ > tests/test-layers.c | 6 ++++++ > 13 files changed, 109 insertions(+)
> +++ b/server/connections.c
> @@ -84,6 +84,7 @@ struct connection {
> bool can_trim;
> bool can_zero;
> bool can_fua;
> + bool can_multi_conn;
> bool using_tls;
>
> int sockin, sockout;
> @@ -463,6 +464,14 @@ compute_eflags (struct connection *conn, uint16_t *flags)
> conn->is_rotational = true;
> }
>
> + fl = backend->can_multi_conn (backend, conn);
> + if (fl == -1)
> + return -1;
> + if (fl) {
> + eflags |= NBD_FLAG_CAN_MULTI_CONN;
> + conn->can_multi_conn = true;
> + }
One more tweak: we should never advertise multi-conn if we are
serializing connections (a client attempting a second connection will
block on that connection, which defeats the point). I'm applying this
followup:
+++ b/server/protocol-handshake.c
@@ -101,12 +101,16 @@ protocol_compute_eflags (struct connection *conn,
uint16_t *flags)
conn->is_rotational = true;
}
- fl = backend->can_multi_conn (backend, conn);
- if (fl == -1)
- return -1;
- if (fl) {
- eflags |= NBD_FLAG_CAN_MULTI_CONN;
- conn->can_multi_conn = true;
+ /* multi-conn is useless if parallel connections are not allowed */
+ if (backend->thread_model (backend) >
+ NBDKIT_THREAD_MODEL_SERIALIZE_CONNECTIONS) {
+ fl = backend->can_multi_conn (backend, conn);
+ if (fl == -1)
+ return -1;
+ if (fl) {
+ eflags |= NBD_FLAG_CAN_MULTI_CONN;
+ conn->can_multi_conn = true;
+ }
}
fl = backend->can_cache (backend, conn);
--
2.20.1
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
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