Re: [Qemu-devel] KVM call agenda for October 25

[Markus Armbruster]

Paolo Bonzini <address@hidden> writes:

> On 10/26/2011 10:48 AM, Markus Armbruster wrote:
>> Sector size is a device property.
>>
>> If the user asks for a 4K sector disk, and the backend can't support
>> that, we need to reject the configuration.  Just like we reject
>> read-only backends for read/write disks.
>
> Isn't it the other way round, i.e. the user asks for a 512-byte sector
> disk (i.e. the default) with cache=none but the disk has 4k sectors?

Let me rephrase: If the user asks for a FOO disk, and the backend can't
support that, we need to reject the configuration.  Just like we reject
read-only backends for read/write disks.

> We're basically saying "choose between NFS and migration if you have
> 4k sector disks but your guest doesn't support them".  Understandable
> perhaps, but not exactly kind, and virtualization is also about
> shielding from this kind of hardware dependency even at the cost of
> performance.  QEMU should just warn about performance degradations,
> erroring out would be a policy decision that should be up to
> management.

I don't have strong opinions on that.

>> It's okay to default device properties to some backend-dependent value,
>> if that improves usability.
>
> On the other hand, not all guests support 4k-sectors properly.

You can't pick perfect defaults for all conceivable guests.  Life's
tough.