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

[Avi Kivity <avi@redhat.com>]

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.