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Date: Thu, 20 May 2010 17:24:31 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH] QEMU: Change default disk caching to
	nocache
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To: Jes.Sorensen@redhat.com
Cc: hch@infradead.org, aliguori@us.ibm.com, qemu-devel@nongnu.org, crobinso@redhat.com

On 05/20/2010 11:32 AM, Jes.Sorensen@redhat.com wrote:
> +        if (bdrv_flags & BDRV_O_NOCACHE) {
> +            fprintf(stderr, "qemu: failed to open disk image %s as "
> +                    "nocache (O_DIRECT) retrying as write-back\n", file);
> +            bdrv_flags &= BDRV_O_NOCACHE;

Missing ~ here.

> +            bdrv_flags |= BDRV_O_CACHE_WB;
> +            if (bdrv_open(dinfo->bdrv, file, bdrv_flags, drv)<  0)
> +                goto error_open;
> +        } else {

I think the retry should be done silently if no cache= option is given. 
  That is cache=none will be the default but:

- if it is not specified and not supported by the image, fall back to 
writeback with no warning.  However, this is just a QoI issue and can be 
fixed later.

- if it is specified and not supported by the image, either fall back to 
writeback with a warning, or fail altogether.  The former would be a 
change in behavior, so it has to be documented somewhere if it changes.

Or maybe add BDRV_O_CACHE_WT and let the backend decide the default?

Paolo