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

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH v5 0/8] Add metadata overlap checks
Date: Fri, 13 Sep 2013 14:37:39 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130805 Thunderbird/17.0.8

On 2013-09-13 14:29, Eric Blake wrote:
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
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.
I have done some short tests here (I think just writing some GB of data to a qcow2 image on a ramdisk should suffice; maybe taking a couple of snapshots to increase the number of inactive L1 tables (although the active L2 tables should affect the results most anyway)) and I don't see a difference so far. I will continue those tests at home where I have more RAM.

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

Yes, that's true.


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