Re: [linux-lvm] migrating a thick LV to a thin LV

[Marian Csontos <mcsontos@redhat.com>]

On 03/12/2014 01:01 AM, Paul B. Henson wrote:
> I'm looking at converting a system to using thin provisioning, and have a
> handful of existing fixed size LV's I want to migrate to the thinpool.
> Ideally, after the migration they would also be thinly provisioned, only
> taking up the amount of space they are actually using, not their existing
> size. All of them have ext4 filesystems on them.
>
> One option would be to use resize2fs to shrink the filesystems down to their
> bare minimum, resize the existing LV to that, dd it onto a thin LV of the
> original size, and then run resize2fs again to bring the filesystem back to
> full capacity without any blocks wasted.
>
> However, I was wondering if I just dd'd the existing LV onto a thin LV of
> the same size, and then ran fstrim, would that release the unused space? I
> see that thin pools support passing down discards to the underlying device,
> but it's not clear whether they also process them internally, freeing up
> space in the thinpool that is no longer in use by the thin LV.

Yes, discarded blocks in the thin-pool data device which are not 
referenced from other thin-volumes (snapshots) are released regardless 
whether the device at the bottom supports discards or not.

-- Martian

>
> Any thoughts?
>
> Thanks.
>
>
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