[Libguestfs] [PATCH libnbd 2/3] Revert "Revert "copy: Always allow multiple connections""

Enable again multiple connections. When using qemu-nbd users need to use: qemu-nbd --shared=4 ... Otherwise nbdcopy will deadlock trying to open 4 connections to qemu-nbd. This reverts commit fb42056929e9177e0de2603e2f430a8111ae7a5f. Signed-off-by: Nir Soffer <nsoffer(a)redhat.com&gt; --- copy/main.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-) diff --git a/copy/main.c b/copy/main.c index b9dbe1d..70a4e11 100644 --- a/copy/main.c +++ b/copy/main.c @@ -321,10 +321,6 @@ main (int argc, char *argv[]) exit (EXIT_FAILURE); } - /* If multi-conn is not supported, force connections to 1. */ - if (! src->ops->can_multi_conn (src) || ! dst->ops->can_multi_conn (dst)) - connections = 1; - /* Calculate the number of threads from the number of connections. */ if (threads == 0) { long t; @@ -379,15 +375,26 @@ main (int argc, char *argv[]) synchronous ? "true" : "false"); } - /* If multi-conn is enabled on either side, then at this point we + /* If using multiple connections, then at this point we * need to ask the backend to open the extra connections. + * + * Using multiple connections when the server does not report + * can_multi_conn is not safe in general, but is safe for the special + * access pattern in nbdcopy. + * + * - We split the image to 128m segments, and every segment is written + * by single worker. + * + * - Writes do not overlap, and are never partial block that another + * worker may modify at the same time. + * + * - We never read from the destination so we don't have caching + * synchronization issue between clients. */ if (connections > 1) { assert (threads == connections); - if (src->ops->can_multi_conn (src)) - src->ops->start_multi_conn (src); - if (dst->ops->can_multi_conn (dst)) - dst->ops->start_multi_conn (dst); + src->ops->start_multi_conn (src); + dst->ops->start_multi_conn (dst); } /* If the source is NBD and we couldn't negotiate meta -- 2.26.3