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Date: Sun, 30 Nov 2008 16:56:23 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [5839] qemu-img: open files with cache=writeback
References: <E1L6ris-0005Tm-5l@cvs.savannah.gnu.org>
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To: qemu-devel@nongnu.org

Aurelien Jarno wrote:
> Revision: 5839
>           http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=5839
> Author:   aurel32
> Date:     2008-11-30 19:12:49 +0000 (Sun, 30 Nov 2008)
>
> Log Message:
> -----------
> qemu-img: open files with cache=writeback
>
> Data integrity is not important in qemu-img, so open the files with
> cache=writeback. This fixes the performance regression seen with qemu-img
> since revision 5485, and most particularly with the qcow2 format.
>
> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
>   

My thinking was to just make qcow2 always default to cache=writeback.  I 
posted an RFC but received no feedback.  I guess that means noone 
disagreed :-)

Regards,

Anthony Liguori

> Revision Links:
> --------------
>     http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=5485
>
> Modified Paths:
> --------------
>     trunk/qemu-img.c
>
> Modified: trunk/qemu-img.c
> ===================================================================
> --- trunk/qemu-img.c	2008-11-30 17:54:28 UTC (rev 5838)
> +++ trunk/qemu-img.c	2008-11-30 19:12:49 UTC (rev 5839)
> @@ -30,6 +30,9 @@
>  #include <windows.h>
>  #endif
>  
> +/* Default to cache=writeback as data integrity is not important for qemu-tcg. */
> +#define BRDV_O_FLAGS BDRV_O_CACHE_WB
> +
>  static void __attribute__((noreturn)) error(const char *fmt, ...)
>  {
>      va_list ap;
> @@ -186,7 +189,7 @@
>      } else {
>          drv = NULL;
>      }
> -    if (bdrv_open2(bs, filename, 0, drv) < 0) {
> +    if (bdrv_open2(bs, filename, BRDV_O_FLAGS, drv) < 0) {
>          error("Could not open '%s'", filename);
>      }
>      if (bdrv_is_encrypted(bs)) {
> @@ -317,7 +320,7 @@
>      } else {
>          drv = NULL;
>      }
> -    if (bdrv_open2(bs, filename, 0, drv) < 0) {
> +    if (bdrv_open2(bs, filename, BRDV_O_FLAGS, drv) < 0) {
>          error("Could not open '%s'", filename);
>      }
>      ret = bdrv_commit(bs);
> @@ -691,7 +694,7 @@
>      } else {
>          drv = NULL;
>      }
> -    if (bdrv_open2(bs, filename, 0, drv) < 0) {
> +    if (bdrv_open2(bs, filename, BRDV_O_FLAGS, drv) < 0) {
>          error("Could not open '%s'", filename);
>      }
>      bdrv_get_format(bs, fmt_name, sizeof(fmt_name));
>
>
>
>
>