Re: [Qemu-devel] [PATCH v12 03/14] qcow2: Optimize bdrv_make_empty()

[Eric Blake <eblake@redhat.com>]

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On 10/10/2014 06:32 AM, Eric Blake wrote:
> On 08/26/2014 03:36 PM, Max Reitz wrote:
>> bdrv_make_empty() is currently only called if the current image
>> represents an external snapshot that has been committed to its base
>> image; it is therefore unlikely to have internal snapshots. In this
>> case, bdrv_make_empty() can be greatly sped up by emptying the L1 and
>> refcount table (while having the dirty flag set) and creating a trivial
>> refcount structure.
>>
>> If there are snapshots, fall back to the simple implementation (discard
>> all clusters).

Brain-wave...


>> +        /* Refcounts will be broken utterly */
>> +        ret = qcow2_mark_dirty(bs);
> 
> qcow2_mark_dirty does assert(s->qcow_version >= 3).  But this code can
> be reached when committing a qcow2 0.10 compat-level file, right?

What if you use the fallback of the slower code on compat=0.10 files?
It's no worse than it has always been, and after all, this patch is
about adding an optimization, not adding new behavior.  It's okay if the
optimization is only usable on compat=1.1 files.

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


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