[lvm-devel] master - python-lvm: Make second lv.snapshot() argument optional

[Zdenek Kabelac]

Dne 16.12.2012 03:27, Andy Grover napsal(a):
> On 12/15/2012 01:34 PM, Zdenek Kabelac wrote:
>> Dne 14.12.2012 23:11, Andy Grover napsal(a):
>>> If no size is given, size defaults to 0, which in lvm_lv_snapshot will
>>> allocate extents equal to the original LV be allocated for the new
>>> snapshot.
>>>
>>
>>
>> Possibly not the ideal solution here.
>>
>> 1) we have some clash with command line 'API'
>>     user creates snapshot of thin volume
>>        - without giving size he get thin volume snaphost.
>>          lvcreate -s
>>        - with size specified he gets 'old' snapshot
>>          (snapshot outside of the pool)
>>          lvcreate -s -l|-L
>
> Makes sense. So should we do the same for the Python API -- size = None
> -> thin snap, size = >=0 -> 'old' snap with the given size? OR might we
> even be OK with just supporting 'new' snaps, and get rid of the size
> parameter altogether?
>

The 'old-snaps' should/will remain supported - since you may want to snap
data outside of the pool - i.e. you want limit maximum size the snapshot 
takes. Or you have very big 'chunk size' of the thin pool - and you may
want to take snapshot of the volume in that pool - so use of snapshot
with significantly smaller chunks size is preferable.

'zero' size snapshot (>=0) - I think this case is actually unsupportable - the 
minimum size for snapshot is I think 2 chunks (1 for data 1 for metadata) - so 
anything smaller should be rejected by metadata validator -  so I think  0 
should be equal to 'None'.

Zdenek