Re: [Qemu-devel] [PATCH v4 2/5] qcow2: Document some maximum size constraints

[Eric Blake <eblake@redhat.com>]

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On 04/13/2018 12:08 PM, Max Reitz wrote:

>>>> With 512 byte clusters and 64 bit refcount entries I still get 8 PB,
>>>> way over what's limited by the L1/L2 tables (128 GB).
>>>
>>> Do I need to make any modifications to the sentence, then?
>>
>> I guess what surprised me the first time that I read it was that it
>> suggests that this has to be taken into account when calculating the
>> physical limits of an image, while in practice it can be ignored.
>>
>> You could say something like 
>>
>>   Although the larger the cluster size, the larger the offsets that can
>>   be covered by the refcount table, in practice these limits cannot be
>>   reached because they are larger than the ones imposed by other data
>>   structures.
> 
> Are there any updates here?  I guess I personally would just drop the
> whole paragraph, because I think it really doesn't matter...

Yeah, I need to post a v5 of this series now that 2.13 is nearly open.

> 
> Also note that the maximum file size of ext4 is 16 PB (for 4 kB blocks).
>  OK, it's bigger for XFS, but that still gives some perspective.
> 
> Also, long before anyone is going to complain about the specification
> failing to mention that limit, they are going to complain that qemu
> refuses to open their image (because of its limit on the reftable size).
> 
> Max
> 
>> although I'm sure that you can come up with a better wording than mine :)
>>
>> Berto
>>
> 
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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