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Date: Tue, 14 Oct 2008 19:15:21 +0200
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
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To: qemu-devel@nongnu.org
Cc: Chris Wright <chrisw@redhat.com>, Mark McLoughlin <markmc@redhat.com>, kvm-devel <kvm-devel@lists.sourceforge.net>, Laurent Vivier <Laurent.Vivier@bull.net>, Ryan Harper <ryanh@us.ibm.com>, Gerd Hoffmann <kraxel@redhat.com>

Jamie Lokier wrote:
> However, should the effect of the guest turning off the IDE disk write
> cache perhaps be identical to the guest issuing IDE cache flush commands
> following every IDE write?
>
> This could mean the host calling fdatasync, or fsync, or using
> O_DSYNC, or O_DIRECT - whatever the host does for IDE flush cache.
>
> What this means _exactly_ for data integrity is outside of qemu's
> control and is a user & host configuration issue.  But qemu could
> provide consistency at least.
>   

We should completely ignore the guest IDE write cache.  It was brought
into life by the deficiencies of IDE which presented the user with an
impossible tradeoff -- you can choose between data loss and horrible
performance.  Since modern hardware doesn't require this tradeoff, there
is no reason to force the user to make these choices.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.