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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: Fri, 10 Oct 2008 12:26:22 +0200
User-agent: Thunderbird (X11/20080723)

Daniel P. Berrange wrote:
There are (at least) three usage models for qemu:

- OS development tool
- casual or client-side virtualization
- server partitioning

The last two uses are almost always in conjunction with a hypervisor.

When using qemu as an OS development tool, data integrity is not very important. On the other hand, performance and caching are, especially as the guest is likely to be restarted multiple times so the guest page cache is of limited value. For this use model the current default (write back cache) is fine.

It is a myth that developers dont' care about data consistency / crash
safety. I've lost countless guest VMs to corruption when my host OS crashed & its just a waste of my time. Given the choice between likely-to-corrupt and not-likely-to-corrupt, even developers will want the latter.

There are other data integrity solutions for developers, like backups (unlikely, I know) or -snapshot.

Absoutely agree that the default should be safe. I don't have enough knowledge to say whether O_DIRECT/O_DSYNC is best - which also implies
we should choose the best setting by default, because we can't expect
users to know the tradeoffs either.

The fact that there are different use models for qemu implies that the default must be chosen at some higher level than qemu code itself. It might be done using /etc/qemu or ~/.qemu, or at the management interface, but there is no best setting for qemu itself.

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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