nfs: Support BDRV_REQ_ZERO_WRITE for truncate

Our .bdrv_has_zero_init_truncate returns 1 if we detect that the OS
always 0-fills; we can use that same knowledge to implement
BDRV_REQ_ZERO_WRITE by ignoring it when the OS gives it to us for
free.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200428202905.770727-4-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/block/nfs.c b/block/nfs.c
index 385d756e1d4484da1eabc429a93da8499fa7b526..76daa7c9f60dc4e99d66ca99dbc8c51d85f1cf40 100644 (file)
--- a/block/nfs.c
+++ b/block/nfs.c
@@ -620,6 +620,9 @@ static int nfs_file_open(BlockDriverState *bs, QDict *options, int flags,
     }
 
     bs->total_sectors = ret;
+    if (client->has_zero_init) {
+        bs->supported_truncate_flags = BDRV_REQ_ZERO_WRITE;
+    }
     return 0;
 }