[Qemu-devel] [PATCH v2] qcow2: Flush image after creation

[Qemu-devel] [PATCH v2] qcow2: Flush image after creation

[Max Reitz]

Opening the qcow2 image with BDRV_O_NO_FLUSH prevents any flushes during
the image creation. This means that the image has not yet been flushed
to disk when qemu-img create exits. This flush is delayed until the next
operation on the image involving opening it without BDRV_O_NO_FLUSH and
closing (or directly flushing) it. For large images and/or images with a
small cluster size and preallocated metadata, this flush may take a
significant amount of time and may occur unexpectedly.

Reopening the image without BDRV_O_NO_FLUSH right before the end of
qcow2_create2() results in hoisting the potentially costly flush into
the image creation, which is expected to take some time (whereas
successive image operations may be not).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
v2:
 - implemented Kevin's remarks:
   - left ret = 0; in the code
   - added goto out; in error handling of bdrv_open
 - implemented Eric's remarks:
   - changed indentation of bdrv_open arguments
   - s/preponing/hoisting/ in the commit message
---
 block/qcow2.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/block/qcow2.c b/block/qcow2.c
index 01269f9..6e5d98d 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1584,6 +1584,16 @@ static int qcow2_create2(const char *filename, int64_t total_size,
         }
     }
 
+    bdrv_close(bs);
+
+    /* Reopen the image without BDRV_O_NO_FLUSH to flush it before returning */
+    ret = bdrv_open(bs, filename, NULL,
+                    BDRV_O_RDWR | BDRV_O_CACHE_WB, drv, &local_err);
+    if (error_is_set(&local_err)) {
+        error_propagate(errp, local_err);
+        goto out;
+    }
+
     ret = 0;
 out:
     bdrv_unref(bs);
-- 
1.8.3.1

Re: [Qemu-devel] [PATCH v2] qcow2: Flush image after creation

[Kevin Wolf]

Am 24.10.2013 um 20:35 hat Max Reitz geschrieben:
> Opening the qcow2 image with BDRV_O_NO_FLUSH prevents any flushes during
> the image creation. This means that the image has not yet been flushed
> to disk when qemu-img create exits. This flush is delayed until the next
> operation on the image involving opening it without BDRV_O_NO_FLUSH and
> closing (or directly flushing) it. For large images and/or images with a
> small cluster size and preallocated metadata, this flush may take a
> significant amount of time and may occur unexpectedly.
> 
> Reopening the image without BDRV_O_NO_FLUSH right before the end of
> qcow2_create2() results in hoisting the potentially costly flush into
> the image creation, which is expected to take some time (whereas
> successive image operations may be not).
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>

Thanks, applied to the block branch.

Kevin

Re: [Qemu-devel] [PATCH v2] qcow2: Flush image after creation

[Benoît Canet]

Le Thursday 24 Oct 2013 à 20:35:06 (+0200), Max Reitz a écrit :
> Opening the qcow2 image with BDRV_O_NO_FLUSH prevents any flushes during
> the image creation. This means that the image has not yet been flushed
> to disk when qemu-img create exits. This flush is delayed until the next
> operation on the image involving opening it without BDRV_O_NO_FLUSH and
> closing (or directly flushing) it. For large images and/or images with a
> small cluster size and preallocated metadata, this flush may take a
> significant amount of time and may occur unexpectedly.
> 
> Reopening the image without BDRV_O_NO_FLUSH right before the end of
> qcow2_create2() results in hoisting the potentially costly flush into
> the image creation, which is expected to take some time (whereas
> successive image operations may be not).
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> ---
> v2:
>  - implemented Kevin's remarks:
>    - left ret = 0; in the code
>    - added goto out; in error handling of bdrv_open
>  - implemented Eric's remarks:
>    - changed indentation of bdrv_open arguments
>    - s/preponing/hoisting/ in the commit message
> ---
>  block/qcow2.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 01269f9..6e5d98d 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1584,6 +1584,16 @@ static int qcow2_create2(const char *filename, int64_t total_size,
>          }
>      }
>  
> +    bdrv_close(bs);
> +
> +    /* Reopen the image without BDRV_O_NO_FLUSH to flush it before returning */
> +    ret = bdrv_open(bs, filename, NULL,
> +                    BDRV_O_RDWR | BDRV_O_CACHE_WB, drv, &local_err);
> +    if (error_is_set(&local_err)) {
> +        error_propagate(errp, local_err);
> +        goto out;
> +    }
> +
>      ret = 0;
>  out:
>      bdrv_unref(bs);
> -- 
> 1.8.3.1
> 
> 
Reviewed-by: Benoit Canet <benoit@irqsave.net>