Re: [Qemu-devel] Re: [PATCH] QEMU: Change default disk caching to nocache

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/20/2010 10:24 AM, Paolo Bonzini wrote:
> 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?

It used to be that we had a CACHE_DEFAULT which allowed qcow2 to do 
CACHE_WB by default whereas everything else did CACHE_WT.  The same 
technique could be used to let physical devices do NOCACHE by default.

Regards,

Anthony Liguori

> Paolo
>