Re: [Qemu-devel] [PATCH v5 0/8] Add metadata overlap checks

[Eric Blake <eblake@redhat.com>]

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On 09/13/2013 04:23 AM, Max Reitz wrote:
> 
>> The more interesting part is that adding an option always needs thought
>> because once it is exposed, it's an API that is set in stone. And I'm
>> also not sure what the best command line and QMP representations of a
>> bitmask like this are.
> I'd personally add it to the runtime options of qcow2. In addition, I
> propose we add a mechanism to generally amend runtime options at runtime
> through QMP (if there isn't one already which I then am unaware of). I
> don't see why we should just allow the kind of overlap checks performed
> to be changed at runtime, but not, for instance, whether lazy refcounts
> should be used (except the latter would be a bit harder to implement, I
> guess).

Indeed - I was asking more to spark conversation, and not to necessarily
state that we need it (again, without benchmark numbers, it's hard to
state whether there's enough timing difference for it to even matter
that someone would WANT a runtime tuning).  IF we implement runtime
tuning, we're stuck supporting it.

> 
> About the representation: The discard behavior is basically a bitfield
> already and gives us therefore one possible representation (which is,
> just using a single boolean per structure, named something like
> "overlap-check.active-l1" etc.). In QMP we could probably also use a
> dict, but then again, this is a decision to be made when generally
> allowing modification of the qcow2 runtime options through QMP (in my
> opinion). And finally, we could obviously just use an integer to
> represent the mask.

Implement it only as a struct of bools.  A raw 'int' requires the caller
to have too much internal knowledge of which bools map to which bit
positions, and furthermore prevents you from ever changing bit positions.

> 
> I think, we should first take care of the command line interface and
> about QMP later (that is, if you agree on generally allowing
> modification of the qcow2 runtime options through QMP). There, we could
> offer both one boolean per mask element and an integer option, probably
> the boolean flags taking precedence.

I'm fine if it is JUST a command-line parameter (all-or-nothing, turned
on when you boot qemu, and not something we can be changing on the fly).
 But if we ever do want live changing via QMP, do NOT expose it as a raw
int, but only as named bools.

> 
> The flags are nice for users who want an "easily" comprehensible
> interface, the masked integer is better for those who prefer a short
> representation.

Short representations that lock us into a particular implementation are bad.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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