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Re: [Qemu-devel] [RFC] Disk integrity in QEMU

From: Avi Kivity
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
Date: Tue, 14 Oct 2008 19:15:21 +0200
User-agent: Thunderbird (X11/20080723)

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 

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