They switched the default cache to writeback in qemu 1.2 which is shipped with F18.
F17 shipped with qemu version 1.0.1 which I think defaults to writethrough.
If you don't change the command I'd think you should get writeback automatically
after the upgrade.
----- Original Message -----
From: Jonathan Masters <jcm(a)redhat.com>
To: Jerry James <loganjerry(a)gmail.com>
Cc: Fedora ARM <arm(a)lists.fedoraproject.org>
Sent: Friday, January 11, 2013 2:12 PM
Subject: Re: [fedora-arm] Write-back vs. write-through for VMs
Hi Jerry,
I think you make a good recommendation, explaining the issue of write back via
cache vs. immediately hitting the disk. I also think your mail explains why we
should not change the default. Instead, lets change the documentation, to
explain that those not needing consistency can use this performance
optimization.
Jon.
--
Sent from my iPad
On Jan 11, 2013, at 10:42, Jerry James <loganjerry(a)gmail.com> wrote:
> I've been using the vexpress image with QEMU to try to track down some
> ARM-specific problems in two of my packages. The VM was unbearably
> slow at first (by which I mean that a simple "yum upgrade" took
over 6
> hours), until I changed:
>
> -sd "$IMAGE"
>
> to:
>
> -drive if=sd,cache=writeback,file="$IMAGE"
>
> in boot-vexpress and boot-vexpress+x. I'm well aware of the danger of
> using writeback, but ... it's a VM. With the script as shipped, my
> hard drive light was on constantly, and the entire host was slowed
> down dramatically due to insufficient disk bandwidth. With writeback
> on, I can re-create the VM and reinstall the packages of interest in
> very little time, certainly less than even simple operations were
> taking with write-through on. Is there some reason why changing the
> scripts to use writeback would not be a good idea?
>
> Regards,
> --
> Jerry James
>
http://www.jamezone.org/
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>
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