On Wed, May 26, 2010 at 09:04:32PM +0200, Alexander Graf wrote:
> When using -snapshot we don't care about data integrity of the cow file
> at all, so let's disable flushing there and squeeze out the last drop
> of performance we could possibly get.
>
> Signed-off-by: Alexander Graf <ag...@suse.de>
Thanks, applied.
> ---
> qemu-options.hx | 6 +++---
> vl.c | 4 ++--
> 2 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/qemu-options.hx b/qemu-options.hx
> index b389f36..6db6d91 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -165,8 +165,7 @@ Specify the controller's PCI address (if=virtio only).
> By default, writeback caching is used for all block devices. It will report
> data writes as completed as soon as the data is present in the host page
> cache. This is safe as long as you trust your host. If your host crashes
> -or loses power, then the guest may experience data corruption. When using
> -the @option{-snapshot} option, writeback caching is always used.
> +or loses power, then the guest may experience data corruption.
>
> Writethrough caching means that the host page cache will be used to read
> and write data but write notification will be sent to the guest only when
> @@ -184,7 +183,8 @@ In case you don't care about data integrity over host
> failures, use
> cache=unsafe. This option tells qemu that it never needs to write any data
> to the disk but can instead keeps things in cache. If anything goes wrong,
> like your host losing power, the disk storage getting disconnected
> accidently,
> -etc. you're image will most probably be rendered unusable.
> +etc. you're image will most probably be rendered unusable. When using
> +the @option{-snapshot} option, unsafe caching is always used.
>
> Instead of @option{-cdrom} you can use:
> @example
> diff --git a/vl.c b/vl.c
> index a70e487..21bd808 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -1108,9 +1108,9 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque,
> return NULL;
> }
> if (snapshot) {
> - /* always use write-back with snapshot */
> + /* always use cache=unsafe with snapshot */
> bdrv_flags &= ~BDRV_O_CACHE_MASK;
> - bdrv_flags |= (BDRV_O_SNAPSHOT|BDRV_O_CACHE_WB);
> + bdrv_flags |= (BDRV_O_SNAPSHOT|BDRV_O_CACHE_WB|BDRV_O_NO_FLUSH);
> }
>
> if (media == MEDIA_CDROM) {
> --
> 1.6.0.2
>
>
--
Aurelien Jarno GPG: 1024D/F1BCDB73
aurel...@aurel32.net http://www.aurel32.net
