[linux-lvm] determining snapshot overhead

[Zdenek Kabelac]

Dne 14.1.2015 v 11:18 Bram Klein Gunnewiek napsal(a):
> Thanks for your reply. I tested it out and it does not seem to work (lvm2
> version used is 2.02.98-6ubuntu2):
>

yep - too old for this feature being supported.


> lvcreate -n testvolume3 -L 1G vg
> lvcreate -n testvolume3_snapshot -s /dev/vg/testvolume3 -l100%ORIGIN
> dd if=/dev/zero of=/dev/vg/testvolume3 bs=1G count=1
>
> After this the snapshot is invalid:
>
> testvolume3_snapshot vg swi-I-s--   1.00g      testvolume3 100.00
>
> I checked out the lvm2 source code and found the methods cow_max_extents and
> _cow_max_size  in snapshot_manip.c. Do these methods calculate the exact max
> size of the snapshot meta data?

yes.

It's probably worth to note there has been some related kernel fixes for old 
snaps - otherwise there has been theoretical path to create much bigger 
snapshots then the origin size was - though it's  nearly impossible to reach 
such state.

Zdenek




>
> On 01/14/2015 09:34 AM, Zdenek Kabelac wrote:
>> Dne 14.1.2015 v 08:24 Bram Klein Gunnewiek napsal(a):
>>> Hello,
>>>
>>> I am trying to create snapshots of thick volumes that can't break because they
>>> overflow (thin volumes are not an option right now). I can't find information
>>> on how to calculate (in advance) the metadata overhead of a snapshot when all
>>> blocks in the origin volume are changed (e.g. the snapshot is 100% full). Is
>>> there a way to do this?
>>
>>
>> Recent versions of lvm2  (let's say >= 111) are able to determine this size
>> automatically - just use: 'lvcreate -s -l100%ORIGIN vg/origin'
>>
>> Regards
>>
>> Zdenek
>>
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>
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