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Re: [Qemu-devel] [PATCH v2] xen_disk: support "direct-io-safe" backend o
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH v2] xen_disk: support "direct-io-safe" backend option |
Date: |
Fri, 28 Jun 2013 09:56:59 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6 |
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