Re: [Libguestfs] [nbdkit PATCH 3/3] plugins: Add .can_fast_zero hook

Allow plugins to affect the handling of the new NBD_CMD_FLAG_FAST_ZERO flag, then update affected plugins. In particular, in-memory plugins are always fast; the full plugin is better served by omitting .zero and relying on .pwrite fallback; nbd takes advantage of libnbd extensions proposed in parallel to pass through support; and v2 language bindings expose the choice to their scripts.

+++ b/server/plugins.c @@ -404,11 +404,25 @@ plugin_can_zero (struct backend *b, struct connection *conn) static int plugin_can_fast_zero (struct backend *b, struct connection *conn) { + struct backend_plugin *p = container_of (b, struct backend_plugin, backend); + int r; + assert (connection_get_handle (conn, 0)); debug ("can_fast_zero"); - return 0; /* Upcoming patch will actually add support. */ + if (p->plugin.can_fast_zero) + return p->plugin.can_fast_zero (connection_get_handle (conn, 0)); + /* Advertise support for fast zeroes if no .zero or .can_zero is + * false: in those cases, we fail fast instead of using .pwrite. + * This also works when v1 plugin has only ._zero_old. + */ + if (p->plugin.zero == NULL) + return 1; + r = plugin_can_zero (b, conn); + if (r == -1) + return -1; + return !r; }