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Use writeback caching by default with qcow2 

qcow2 writes a cluster reference count on every cluster update.  This causes
performance to crater when using anything but cache=writeback.  This is most
noticeable when using savevm.  Right now, qcow2 isn't a reliable format
regardless of the type of cache your using because metadata is not updated in
the correct order.  Considering this, I think it's somewhat reasonable to use
writeback caching by default with qcow2 files.

It at least avoids the massive performance regression for users until we sort
out the issues in qcow2. 

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>



git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5879 c046a42c-6fe2-441c-8c8c-71466251a162
This commit is contained in:
aliguori 2008-12-04 21:39:21 +00:00
parent 0cd2df75a2
commit 4dc822d726
4 changed files with 19 additions and 2 deletions
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8
block-qcow2.c
View File

@ -189,6 +189,14 @@ static int qcow_open(BlockDriverState *bs, const char *filename, int flags)
int len, i, shift, ret;
QCowHeader header;
/* 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;

3
block.h
View File

@ -49,8 +49,9 @@ 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)
#define BDRV_O_CACHE_MASK (BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_CACHE_DEF)
void bdrv_info(void);
void bdrv_info_stats(void);

6
qemu-doc.texi
View File

@ -289,6 +289,12 @@ The host page can be avoided entirely with @option{cache=none}. This will
attempt to do disk IO directly to the guests memory. QEMU may still perform
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.
Instead of @option{-cdrom} you can use:
@example
qemu -drive file=file,index=2,media=cdrom

4
vl.c
View File

@ -2242,7 +2242,7 @@ static int drive_init(struct drive_opt *arg, int snapshot,
unit_id = -1;
translation = BIOS_ATA_TRANSLATION_AUTO;
index = -1;
cache = 1;
cache = 3;
if (machine->use_scsi) {
type = IF_SCSI;
@ -2517,6 +2517,8 @@ static int drive_init(struct drive_opt *arg, int snapshot,
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 || qemu_key_check(bdrv, file)) {
fprintf(stderr, "qemu: could not open disk image %s\n",
file);