[Qemu-devel] [PATCH 5/7] 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.
Unfortunately, FALLOC_FL_ZERO_RANGE is supported on really modern systems
and only for a couple of filesystems. FALLOC_FL_PUNCH_HOLE is much more
mature.

The sequence of 2 operations FALLOC_FL_PUNCH_HOLE and 0 is necessary due
to the following reasons:
- FALLOC_FL_PUNCH_HOLE creates a hole in the file, the file becomes
  sparse. In order to retain original functionality we must allocate
  disk space afterwards. This is done using fallocate(0) call
- fallocate(0) without preceeding FALLOC_FL_PUNCH_HOLE will do nothing
  if called above already allocated areas of the file, i.e. the content
  will not be zeroed

This should increase the performance a bit for not-so-modern kernels.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Max Reitz <mreitz@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Peter Lieven <pl@kamp.de>
CC: Fam Zheng <famz@redhat.com>
---
 block/raw-posix.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/block/raw-posix.c b/block/raw-posix.c
index 5a777e7..2e24829 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -965,6 +965,21 @@ static ssize_t handle_aiocb_write_zeroes(RawPosixAIOData *aiocb)
     }
 #endif
 
+#ifdef CONFIG_FALLOCATE_PUNCH_HOLE
+    if (s->has_discard) {
+        int ret = do_fallocate(s->fd,
+                               FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+                               aiocb->aio_offset, aiocb->aio_nbytes);
+        if (ret == 0) {
+            ret = do_fallocate(s->fd, 0, aiocb->aio_offset, aiocb->aio_nbytes);
+        }
+        if (ret != -ENOTSUP) {
+            return ret;
+        }
+        s->has_discard = false;
+    }
+#endif
+
     return -ENOTSUP;
 }
 
-- 
1.9.1