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qcow2: Make cache=writethrough default 

The performance of qcow2 has improved meanwhile, so we don't need to
special-case it any more. Switch the default to write-through caching
like all other block drivers.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
This commit is contained in:
Kevin Wolf 2009-06-30 13:06:04 +02:00 committed by Anthony Liguori
parent fc5b81d1f6
commit 0aa217e461
4 changed files with 3 additions and 16 deletions
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3
block.h
View File

@ -37,9 +37,8 @@ typedef struct QEMUSnapshotInfo {
bdrv_file_open()) */
#define BDRV_O_NOCACHE 0x0020 /* do not use the host page cache */
#define BDRV_O_CACHE_WB 0x0040 /* use write-back caching */
#define BDRV_O_CACHE_DEF 0x0080 /* use default caching */
#define BDRV_O_CACHE_MASK (BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_CACHE_DEF)
#define BDRV_O_CACHE_MASK (BDRV_O_NOCACHE | BDRV_O_CACHE_WB)
void bdrv_info(Monitor *mon);
void bdrv_info_stats(Monitor *mon);

8
block/qcow2.c
View File

@ -145,14 +145,6 @@ static int qcow_open(BlockDriverState *bs, const char *filename, int flags)
QCowHeader header;
uint64_t ext_end;
/* Performance is terrible right now with cache=writethrough due mainly
* to reference count updates. If the user does not explicitly specify
* a caching type, force to writeback caching.
*/
if ((flags & BDRV_O_CACHE_DEF)) {
flags |= BDRV_O_CACHE_WB;
flags &= ~BDRV_O_CACHE_DEF;
}
ret = bdrv_file_open(&s->hd, filename, flags);
if (ret < 0)
return ret;

4
qemu-options.hx
View File

@ -148,9 +148,7 @@ an internal copy of the data.
Some block drivers perform badly with @option{cache=writethrough}, most notably,
qcow2. If performance is more important than correctness,
@option{cache=writeback} should be used with qcow2. By default, if no explicit
caching is specified for a qcow2 disk image, @option{cache=writeback} will be
used. For all other disk types, @option{cache=writethrough} is the default.
@option{cache=writeback} should be used with qcow2.
Instead of @option{-cdrom} you can use:
@example

4
vl.c
View File

@ -2239,7 +2239,7 @@ int drive_init(struct drive_opt *arg, int snapshot, void *opaque)
unit_id = -1;
translation = BIOS_ATA_TRANSLATION_AUTO;
index = -1;
cache = 3;
cache = 1;
if (machine->use_scsi) {
type = IF_SCSI;
@ -2557,8 +2557,6 @@ int drive_init(struct drive_opt *arg, int snapshot, void *opaque)
bdrv_flags |= BDRV_O_NOCACHE;
else if (cache == 2) /* write-back */
bdrv_flags |= BDRV_O_CACHE_WB;
else if (cache == 3) /* not specified */
bdrv_flags |= BDRV_O_CACHE_DEF;
if (bdrv_open2(bdrv, file, bdrv_flags, drv) < 0) {
fprintf(stderr, "qemu: could not open disk image %s\n",
file);