diff options
authorEric Blake <eblake@redhat.com>2021-02-09 09:27:58 -0600
committerEric Blake <eblake@redhat.com>2021-02-12 07:42:08 -0600
commit582d4210eb2f2ab5baac328fe4b479cd86da1647 (patch)
parenteac92d316351b855ba79eb374dd21cc367f1f9c1 (diff)
qemu-nbd: Use SOMAXCONN for socket listen() backlog
Our default of a backlog of 1 connection is rather puny; it gets in the way when we are explicitly allowing multiple clients (such as qemu-nbd -e N [--shared], or nbd-server-start with its default "max-connections":0 for unlimited), but is even a problem when we stick to qemu-nbd's default of only 1 active client but use -t [--persistent] where a second client can start using the server once the first finishes. While the effects are less noticeable on TCP sockets (since the client can poll() to learn when the server is ready again), it is definitely observable on Unix sockets, where on Linux, a client will fail with EAGAIN and no recourse but to sleep an arbitrary amount of time before retrying if the server backlog is already full. Since QMP nbd-server-start is always persistent, it now always requests a backlog of SOMAXCONN; meanwhile, qemu-nbd will request SOMAXCONN if persistent, otherwise its backlog should be based on the expected number of clients. See https://bugzilla.redhat.com/1925045 for a demonstration of where our low backlog prevents libnbd from connecting as many parallel clients as it wants. Reported-by: Richard W.M. Jones <rjones@redhat.com> Signed-off-by: Eric Blake <eblake@redhat.com> CC: qemu-stable@nongnu.org Message-Id: <20210209152759.209074-2-eblake@redhat.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Signed-off-by: Eric Blake <eblake@redhat.com>
2 files changed, 15 insertions, 2 deletions
diff --git a/blockdev-nbd.c b/blockdev-nbd.c
index d8443d235b..b264620b98 100644
--- a/blockdev-nbd.c
+++ b/blockdev-nbd.c
@@ -134,7 +134,12 @@ void nbd_server_start(SocketAddress *addr, const char *tls_creds,
- if (qio_net_listener_open_sync(nbd_server->listener, addr, 1, errp) < 0) {
+ /*
+ * Because this server is persistent, a backlog of SOMAXCONN is
+ * better than trying to size it to max_connections.
+ */
+ if (qio_net_listener_open_sync(nbd_server->listener, addr, SOMAXCONN,
+ errp) < 0) {
goto error;
diff --git a/qemu-nbd.c b/qemu-nbd.c
index 608c63e82a..1a340ea485 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -964,8 +964,16 @@ int main(int argc, char **argv)
server = qio_net_listener_new();
if (socket_activation == 0) {
+ int backlog;
+ if (persistent) {
+ backlog = SOMAXCONN;
+ } else {
+ backlog = MIN(shared, SOMAXCONN);
+ }
saddr = nbd_build_socket_address(sockpath, bindto, port);
- if (qio_net_listener_open_sync(server, saddr, 1, &local_err) < 0) {
+ if (qio_net_listener_open_sync(server, saddr, backlog,
+ &local_err) < 0) {