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

[Stefan Berger]

> address@hidden 
> Sent by: address@hidden
> 
> 
> 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.

I applied it and tried 'delvm', 'savevm' and 'loadvm'
and at least the performance problems went away. Thanks.

I see some other problems after restoring a VM and
X11 not responding to keyboard inputs or Linux not shutting down properly.
But these may be due to me making some experiments with the 'unreliable'
features of qcow2.

   Stefan

> 
> Regards,
> 
> Anthony Liguori
> 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,
> address@hidden 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);