[Qemu-devel] [PATCH 6/7] block: use fallocate(FALLOC_FL_PUNCH_HOLE) & fallocate(0) to write zeroes

["Denis V. Lunev" <den@openvz.org>]

This sequence works efficiently if FALLOC_FL_ZERO_RANGE is not supported.

Simple fallocate(0) will extend file with zeroes when appropriate in the
middle of the file if there is a hole there and at the end of the file.
Unfortunately fallocate(0) does not drop the content of the file if
there is a data on this offset. Therefore to make the situation consistent
we should drop the data beforehand. This is done using FALLOC_FL_PUNCH_HOLE

This should increase the performance a bit for not-so-modern kernels or for
filesystems which do not support FALLOC_FL_ZERO_RANGE.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/raw-posix.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/block/raw-posix.c b/block/raw-posix.c
index 7866d31..96a8678 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -968,6 +968,23 @@ static ssize_t handle_aiocb_write_zeroes(RawPosixAIOData *aiocb)
 #endif
 
     s->has_write_zeroes = false;
+
+#ifdef CONFIG_FALLOCATE_PUNCH_HOLE
+    if (s->has_discard) {
+        int ret;
+        ret = do_fallocate(s->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+                           aiocb->aio_offset, aiocb->aio_nbytes);
+        if (ret < 0) {
+            if (ret == -ENOTSUP) {
+                s->has_discard = false;
+            }
+            return ret;
+        }
+        return do_fallocate(s->fd, 0, aiocb->aio_offset, aiocb->aio_nbytes);
+    }
+#endif
+
+    s->has_discard = false;
     return -ENOTSUP;
 }
 
-- 
1.9.1