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Re: [Qemu-devel] [PATCH 5/7] block: use fallocate(FALLOC_FL_PUNCH_HOLE)


From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH 5/7] block: use fallocate(FALLOC_FL_PUNCH_HOLE) & fallocate(0) to write zeroes
Date: Thu, 29 Jan 2015 17:40:01 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

On 2015-01-28 at 13:38, Denis V. Lunev wrote:
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 <address@hidden>
CC: Max Reitz <address@hidden>
CC: Kevin Wolf <address@hidden>
CC: Stefan Hajnoczi <address@hidden>
CC: Peter Lieven <address@hidden>
CC: Fam Zheng <address@hidden>
---
  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;
+    }

The problem with putting do_fallocate() there is that if do_fallocate() with FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE works but do_fallocate() without any flags does not, has_discard will be set to false which will render handle_aiocb_discard() useless.

I don't think that that's possible, though (the first do_fallocate() working, but the second returning -ENOTSUP), so here's one rather reluctant:

Reviewed-by: Max Reitz <address@hidden>

+#endif
+
      return -ENOTSUP;
  }




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