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Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive

From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive
Date: Tue, 11 May 2010 08:20:50 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20091209 Fedora/3.0-4.fc12 Lightning/1.0pre Thunderbird/3.0

On 05/11/2010 08:12 AM, Paul Brook wrote:
cache=always (or a more scary name like cache=lie to defend against

    Reads and writes are cached. Guest flushes are ignored.  Useful for
    dumb guests in non-critical environments.
I really don't believe that we should support a cache=lie.  There are
many other obtain the same results.  For instance, mount your guest
filesystem with barrier=0.
Ideally yes. However in practice I suspect this is still a useful option. Is
it even possible to disable barriers in all cases (e.g. NTFS under windows)?

In a production environment it's probably not so useful - you're generally
dealing with long lived, custom configured guests.

In a development environment the rules can be a bit different. For example if
you're testing an OS installer then you really don't want to be passing magic
mount options. If the host machine dies then you don't care about the state of
the guest because you're going to start from scratch anyway.

Then create a mount point on your host and mount the host file system under that mount with barrier=0.

The problem with options added for developers is that those options are very often accidentally used for production.


Anthony Liguori


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