Re: [Qemu-devel] [RFC PATCH v2] Specification for qcow2 version 3

[Stefan Hajnoczi]

On Wed, Oct 12, 2011 at 2:31 PM, Kevin Wolf <address@hidden> wrote:
> Am 12.10.2011 14:51, schrieb Stefan Hajnoczi:
>>> Also a bit in l2 offset to say "there is no l2 table" cause all
>>> clusters in l2 are contiguous so we avoid entirely l2. Obviously this
>>> require an optimization step to detect or create such condition.
>>
>> There are several reserved L1 entry bits which could be used to mark
>> this mode.  This mode severely restricts qcow2 features though: how
>> would snapshots and COW work?  Perhaps by breaking the huge cluster
>> back into an L2 table with individual clusters?  Backing files also
>> cannot be used - unless we extend the sub-clusters approach and also
>> keep a large bitmap with allocated/unallocated/zero information.
>>
>> A mode like this could be used for best performance on local storage,
>> where efficiently image transport (e.g. scp or http) is not required.
>> Actually I think this is reasonable, we could use qemu-img convert to
>> produce a compact qcow2 for export and use the L2-less qcow2 for
>> running the actual VM.
>>
>> Kevin: what do you think about fleshing out this mode instead of 
>> sub-clusters?
>
> I'm hesitant to something like this as it adds quite some complexity and
> I'm not sure if there are practical use cases for it at all.
>
> If you take the current cluster sizes, an L2 table contains 512 MB of
> data, so you would lose any sparseness. You would probably already get
> full allocation just by creating a file system on the image.
>
> But even if you do have a use case where sparseness doesn't matter, the
> effect is very much the same as allowing a 512 MB cluster size and not
> changing any of the qcow2 internals.

I guess I'm thinking of the 512 MB cluster size situation, because
we'd definitely want a cow bitmap in order to keep backing files and
sparseness.

> (What would the use case be? Backing files or snapshots with a COW
> granularity of 512 MB isn't going to fly. That leaves only something
> like encryption.)

COW granularity needs to stay at 64-256 kb since those are reasonable
request sizes for COW.

Stefan