[Qemu-devel] [PATCH] Make cache=unsafe the default for -snapshot

[Alexander Graf]

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 <address@hidden>
---
 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