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Re: [Xen-devel] RFC/PATCH: hdparm tunable IDE write cache for HVM

To: Christian.Limpach@xxxxxxxxxxxx
Subject: Re: [Xen-devel] RFC/PATCH: hdparm tunable IDE write cache for HVM
From: Rik van Riel <riel@xxxxxxxxxx>
Date: Fri, 11 Aug 2006 13:44:51 -0400
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Christian Limpach wrote:
On 8/11/06, Rik van Riel <riel@xxxxxxxxxx> wrote:
qemu 0.8.2 has a flush callback to the storage backends, so now it is
possible to implement hdparm tunable IDE write cache enable/disable for
guest domains, allowing people to pick speed or data consistency on a
case by case basis.

As an added benefit, really large LBA48 IOs will now no longer be broken
up into smaller IOs on the host side.

What do you think?

Looks good, are you confident that you have identified all the places
where a flush is needed?

Yes, bdrv_write() is only called from 2 places in ide.c, and
both of them have a bdrv_flush() call now.

Doesn't qemu do flushes in the first place
and wouldn't it then have the same issues?

Nope, qemu does not seem to do any flushes by itself...

"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan

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