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

[Kevin Wolf]

Am 12.10.2011 16:37, schrieb 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.

But how do you do that without L2 tables? What you're describing
(different sizes for allocation and COW) is exactly what subclusters are
doing. I can't see how switching to 512 MB clusters and a single-level
table can make that work.

Kevin