[linux-lvm] Converting existing thick LVs into thin LVs possible?

[Jeremy Smith]

On Mon, Feb 17, 2014 at 4:00 AM, Zdenek Kabelac <zkabelac at redhat.com> wrote:
> Dne 14.2.2014 20:21, Jeremy Smith napsal(a):
>>
>> I have a system currently using LVM2 that was originally set up on
>> RHEL5 (where thin provisioning was not supported).
>> This system would greatly benefit from the newer thin provisioning
>> features, and so I am investigating upgrading to RHEL6 so that I may
>> take advantage. My question is this: Can I convert existing LVM2
>> "thick" logical volumes into thinly-provisioned volumes on the fly?
>>
>> This page:
>>
>> https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/thinly_provisioned_volume_creation.html
>>
>> lead me to believe that it is possible:
>>   "You can use the --thinpool parameter of the lvconvert command to
>> convert an existing logical volume to a thin volume."
>>
>> with the following:
>>
>>   lvconvert --thinpool vg001/lv1 --poolmetadata vg001/lv2
>
>
>
> Sounds like misunderstanding which needs fix/enhancement of this doc page  -
> we will need to enhance lvconvert command to write big warning about this
> operation.
>
> This conversion is converting  volume to 'data' volume - but it's not about
> preserving content of LV1 as  LV1 - it will serve as en empty pool.
>
> The primary purpose is - you can build 'any' type of LV for data volume
> and 'any' type of LV for metadata volume (i.e. special layout for
> raid/mirror)
> and then you 'create' for these 2 volumes your thin pool - since normally
> you can't create mirrored data & metadata volumes and thin pool in 1
> command,
> and you can't easily select where the metadata should be placed
> (i.e. data on spindle, metadata on SSD).
>
>>
>> Am I doing something wrong here?
>> Or am I going to have to create a new thin pool, new thin logical
>> volumes, and copy the data over?
>
>
> There is not yet support for direct conversion of LV to thin-LV.
> The only currently supported method is to use your  thick-LV as an external
> origin LV for your  thin-LV volume.
>
> i.e. you create your  thin-pool  LV  + and then thin-LV volume with external
> origin that will use original thick-LV  (external origin need to be
> read-only volume)
>
> Here are the conversion steps:
>
> lvcreate -Lsize_of_data_pool -T vg/pool
> lvconvert -T  vg/thick_LV --originname new_thick_LV_name --thinpool  vg/pool
>
> Disadvantage here is you cannot merge changes from thin volume back to
> original thick volume. You can however remove  thin-LV and use again your
> thick-LV as normal volume - i.e. thin-LV is basically snapshot.
>
> Now hint for recovery - if you have not yet written anything to thin-LV from
> newly create thin-pool.  You could restore back to your previous
> configuration
> via   vgcfgrestore --force.
>
> Zdenek
>

Zdenek,

Thank you. I was able to perform migration of some of my LVs, but now
I have ran into a problem: the thin pool metadata has ran out of space
:-(
I am running LVM 2.02.98(2) (2012-10-15) on kernel 3.11.0-18-generic
(Ubuntu server 13.10), which -- as I understand from your talk at
https://www.youtube.com/watch?v=dGax09aw7WI -- does not support live
resizing of the pool metadata.
You mention in that talk that there is a manual offline pool metadata
resize method that can be done on kernels prior to 3.14. I was not
able to find details anywhere on how to accomplish this. Could you
point me in the right direction on how to fix a thin pool (and its
thin logical volumes) that has reached the metadata size limit? It
would be greatly appreciated.

Regards,

-- 
Jeremy Smith
Systems Administrator
eSilo, LLC.
1530 Cypress Drive, Suite H
Jupiter, FL. 33469
561-747-6880 x 412 (Phone)
800-747-9255 x 412 (Toll Free)
866-598-1358 (After Hours)
561-747-6488 (Fax)
jeremy at esilo.com