Re: [Qemu-devel] [PATCH v2] xen_disk: support "direct-io-safe" backend option

[Paolo Bonzini <pbonzini@redhat.com>]

Il 27/06/2013 20:16, Stefano Stabellini ha scritto:
> Support backend option "direct-io-safe".  This is documented as
> follows in the Xen backend specification:
> 
>  * direct-io-safe
>  *      Values:         0/1 (boolean)
>  *      Default Value:  0
>  *
>  *      The underlying storage is not affected by the direct IO memory
>  *      lifetime bug.  See:
>  *        http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html
>  *
>  *      Therefore this option gives the backend permission to use
>  *      O_DIRECT, notwithstanding that bug.
>  *
>  *      That is, if this option is enabled, use of O_DIRECT is safe,
>  *      in circumstances where we would normally have avoided it as a
>  *      workaround for that bug.  This option is not relevant for all
>  *      backends, and even not necessarily supported for those for
>  *      which it is relevant.  A backend which knows that it is not
>  *      affected by the bug can ignore this option.
>  *
>  *      This option doesn't require a backend to use O_DIRECT, so it
>  *      should not be used to try to control the caching behaviour.
> 
> Also, BDRV_O_NATIVE_AIO is ignored if BDRV_O_NOCACHE, so clarify the
> default flags passed to the qemu block layer.
> 
> The original proposal for a "cache" backend option has been dropped
> because it was believed too wide, especially considering that at the
> moment the backend doesn't have a way to tell the toolstack that it is
> capable of supporting it.

Given how rusty my xenstore-fu is, I'll ask the obvious: the frontend
cannot write to it, can it?

Paolo