gwj / at91sam9263 · Commits

Commit 4dc822d726376fd4369089f04eb8605d2f94b74f

Authored by aliguori
1 parent 0cd2df75

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
Inline Side-by-side
Showing 4 changed files with 19 additions and 2 deletions
block-qcow2.c
  View file @4dc822d
... ... @@ -189,6 +189,14 @@ static int qcow_open(BlockDriverState *bs, const char *filename, int flags)
189 189 int len, i, shift, ret;
190 190 QCowHeader header;
191 191  
  192 + /* Performance is terrible right now with cache=writethrough due mainly
  193 + * to reference count updates. If the user does not explicitly specify
  194 + * a caching type, force to writeback caching.
  195 + */
  196 + if ((flags & BDRV_O_CACHE_DEF)) {
  197 + flags |= BDRV_O_CACHE_WB;
  198 + flags &= ~BDRV_O_CACHE_DEF;
  199 + }
192 200 ret = bdrv_file_open(&s->hd, filename, flags);
193 201 if (ret < 0)
194 202 return ret;
... ...
... ... @@ -49,8 +49,9 @@ typedef struct QEMUSnapshotInfo {
49 49 bdrv_file_open()) */
50 50 #define BDRV_O_NOCACHE 0x0020 /* do not use the host page cache */
51 51 #define BDRV_O_CACHE_WB 0x0040 /* use write-back caching */
  52 +#define BDRV_O_CACHE_DEF 0x0080 /* use default caching */
52 53  
53   -#define BDRV_O_CACHE_MASK (BDRV_O_NOCACHE | BDRV_O_CACHE_WB)
  54 +#define BDRV_O_CACHE_MASK (BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_CACHE_DEF)
54 55  
55 56 void bdrv_info(void);
56 57 void bdrv_info_stats(void);
... ...
qemu-doc.texi
  View file @4dc822d
... ... @@ -289,6 +289,12 @@ The host page can be avoided entirely with @option{cache=none}. This will
289 289 attempt to do disk IO directly to the guests memory. QEMU may still perform
290 290 an internal copy of the data.
291 291  
  292 +Some block drivers perform badly with @option{cache=writethrough}, most notably,
  293 +qcow2. If performance is more important than correctness,
  294 +@option{cache=writeback} should be used with qcow2. By default, if no explicit
  295 +caching is specified for a qcow2 disk image, @option{cache=writeback} will be
  296 +used. For all other disk types, @option{cache=writethrough} is the default.
  297 +
292 298 Instead of @option{-cdrom} you can use:
293 299 @example
294 300 qemu -drive file=file,index=2,media=cdrom
... ...
... ... @@ -2242,7 +2242,7 @@ static int drive_init(struct drive_opt *arg, int snapshot,
2242 2242 unit_id = -1;
2243 2243 translation = BIOS_ATA_TRANSLATION_AUTO;
2244 2244 index = -1;
2245   - cache = 1;
  2245 + cache = 3;
2246 2246  
2247 2247 if (machine->use_scsi) {
2248 2248 type = IF_SCSI;
... ... @@ -2517,6 +2517,8 @@ static int drive_init(struct drive_opt *arg, int snapshot,
2517 2517 bdrv_flags |= BDRV_O_NOCACHE;
2518 2518 else if (cache == 2) /* write-back */
2519 2519 bdrv_flags |= BDRV_O_CACHE_WB;
  2520 + else if (cache == 3) /* not specified */
  2521 + bdrv_flags |= BDRV_O_CACHE_DEF;
2520 2522 if (bdrv_open2(bdrv, file, bdrv_flags, drv) < 0 || qemu_key_check(bdrv, file)) {
2521 2523 fprintf(stderr, "qemu: could not open disk image %s\n",
2522 2524 file);
... ...