Re: [bugfix ping2] Re: [PATCH v2 0/2] fix qcow2_can_store_new_dirty_bitmap

[Max Reitz <mreitz@redhat.com>]

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On 10.12.19 09:11, Max Reitz wrote:
> On 09.12.19 23:03, Eric Blake wrote:
>> On 12/9/19 11:58 AM, Max Reitz wrote:
>>> On 09.12.19 17:30, Max Reitz wrote:
>>>> On 02.12.19 15:09, Vladimir Sementsov-Ogievskiy wrote:
>>>>> Hi again!
>>>>>
>>>>> Still forgotten bug-fix :(
>>>>>
>>>>> Is it too late for 4.2?
>>>>
>>>> Sorry. :-/
>>>>
>>>> Yes, I think I just forgot it.  I don’t think it’s too important for
>>>> 4.2, so, well, it isn’t too bad, but...  Sorry.
>>>>
>>>>> I can't imagine better test, and it tests exactly what written in
>>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1712636
>>>>>
>>>>> (Hmm, actually, I doubt that it is real use-case, more probably it's
>>>>> a bug in management layer)
>>>>>
>>>>> So, take this with test or without test, to 4.2 or 5.0.
>>>>
>>>> I was thinking of seeing whether I could write a quicker test, but of
>>>> course we should take the patch either way.
>>>
>>> OK, I give up.  It’s very much possible to create an image with 65535
>>> bitmaps very quickly (like, under a second) outside of qemu, but just
>>> opening it takes 2:30 min (because of the quadratic complexity of
>>> checking whether a bitmap of the same name already exists).
>>
>> Can we fix that to use a hash table for amortized O(1) lookup rather
>> than the current O(n) lookup?
> 
> Not unreasonable, considering that this is probably what we would’ve
> done from the start in any language where hash tables are built in.
> 
> But OTOH when you have 66k bitmaps, you probably have other problems.
> Like, writes being incredibly slow, because all those bitmaps have to be
> updated.
> 
> (Well, you can technically have 99 % of them disabled, but who’d do such
> a thing?)
> 
> ((Maybe I’ll look into it.))

Hmm, now I did.  This gets the test down to 24 s.  Still not sure
whether it’s worth it, though...

Max


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