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

[Anthony Liguori <aliguori@us.ibm.com>]

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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.

Regards,

Anthony Liguori

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Index: vl.c
===================================================================
--- vl.c	(revision 5751)
+++ vl.c	(working copy)
@@ -2225,7 +2225,7 @@
     unit_id = -1;
     translation = BIOS_ATA_TRANSLATION_AUTO;
     index = -1;
-    cache = 1;
+    cache = 3;
 
     if (machine->use_scsi) {
         type = IF_SCSI;
@@ -2496,6 +2496,8 @@
         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);
Index: qemu-doc.texi
===================================================================
--- qemu-doc.texi	(revision 5751)
+++ qemu-doc.texi	(working copy)
@@ -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
Index: block-qcow2.c
===================================================================
--- block-qcow2.c	(revision 5751)
+++ block-qcow2.c	(working copy)
@@ -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;
Index: block.h
===================================================================
--- block.h	(revision 5751)
+++ block.h	(working copy)
@@ -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);