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

[Avi Kivity]

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
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