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Make cache=unsafe the default for -snapshot 

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 <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
This commit is contained in:
Alexander Graf 2010-05-26 21:04:32 +02:00 committed by Aurelien Jarno
parent d63baf92e3
commit c317728872
2 changed files with 5 additions and 5 deletions
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6
qemu-options.hx
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@ -170,8 +170,7 @@ the storage subsystem.
Writeback caching 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
used by default.
corruption.
The host page cache can be avoided entirely with @option{cache=none}. This will
attempt to do disk IO directly to the guests memory. QEMU may still perform
@ -185,7 +184,8 @@ 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

4
vl.c
View File

@ -1105,9 +1105,9 @@ 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) {