[Qemu-devel] [5879] Use writeback caching by default with qcow2

[Qemu-devel] [5879] Use writeback caching by default with qcow2

[Anthony Liguori]

Revision: 5879
          http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=5879
Author:   aliguori
Date:     2008-12-04 21:39:21 +0000 (Thu, 04 Dec 2008)

Log Message:
-----------
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>

Modified Paths:
--------------
    trunk/block-qcow2.c
    trunk/block.h
    trunk/qemu-doc.texi
    trunk/vl.c

Modified: trunk/block-qcow2.c
===================================================================
--- trunk/block-qcow2.c	2008-12-04 21:34:52 UTC (rev 5878)
+++ trunk/block-qcow2.c	2008-12-04 21:39:21 UTC (rev 5879)
@@ -189,6 +189,14 @@
     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;

Modified: trunk/block.h
===================================================================
--- trunk/block.h	2008-12-04 21:34:52 UTC (rev 5878)
+++ trunk/block.h	2008-12-04 21:39:21 UTC (rev 5879)
@@ -49,8 +49,9 @@
                                      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);

Modified: trunk/qemu-doc.texi
===================================================================
--- trunk/qemu-doc.texi	2008-12-04 21:34:52 UTC (rev 5878)
+++ trunk/qemu-doc.texi	2008-12-04 21:39:21 UTC (rev 5879)
@@ -289,6 +289,12 @@
 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

Modified: trunk/vl.c
===================================================================
--- trunk/vl.c	2008-12-04 21:34:52 UTC (rev 5878)
+++ trunk/vl.c	2008-12-04 21:39:21 UTC (rev 5879)
@@ -2242,7 +2242,7 @@
     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 @@
         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);