Re: [linux-lvm] determining snapshot overhead

[Bram Klein Gunnewiek <bram@shockmedia.nl>]

Thanks for your reply. I tested it out and it does not seem to work 
(lvm2 version used is 2.02.98-6ubuntu2):

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?

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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