[linux-lvm] What is the use of thin snapshots if the external origin cannot be set to writable ?

Sreyan Chakravarty sreyan32 at gmail.com
Tue Nov 24 11:59:36 UTC 2020

On Mon, Nov 23, 2020 at 6:34 PM Bryn M. Reeves <bmr at redhat.com> wrote:

> OK - I understand what's going on in your environment now, thanks!
> Unfortunately it's not possible to have a writable external origin when
> using device-mapper thin provisioned snapshots. To be able to write to
> the origin while snapshots exist the origin device must also be a thin
> provisioned logical volume.
> This is explained in more detail in the kernel documentation for the
> thin provisioning targets.
> Documentation/admin-guide/device-mapper/thin-provisioning.rst:
>     External snapshots
>     ------------------
>     You can use an external **read only** device as an origin for a
>     thinly-provisioned volume.  Any read to an unprovisioned area of the
>     thin device will be passed through to the origin.  Writes trigger
>     the allocation of new blocks as usual.
>     One use case for this is VM hosts that want to run guests on
>     thinly-provisioned volumes but have the base image on another device
>     (possibly shared between many VMs).
>     You must not write to the origin device if you use this technique!
>     Of course, you may write to the thin device and take internal snapshots
>     of the thin volume.
> This allows a few niche use cases (like the VM example given), but it's
> not the conventional way of using snapshots with thinp and it does
> restrict what you can do.
> This means that to use thinp snapshots most effectively you must set the
> system up with a thin pool from the start (e.g. using the distro's
> installer to set up the VG).
> >             Command on LV vgfedora/fedora uses options that are invalid
> > with LV parameters: lv_is_external_origin.
> This is correct: currently you cannot make the origin writable since it
> is an external snapshot.
> There is some work going on at the moment that would make device-mapper
> type features more flexible and available in other device types, but
> with the features provided by current tools and the thinp kernel
> support you need to use a thinp device for the snapshot origin too.
> > Is there some sort of resolution ?
> It means re-installing but if the system is set up to use a thin pool
> and thin provisioned logical volumes from the start then you can use
> snapshots without any of the limitations that you've bumped into with
> external origin devices.

Do I have to reinstall my system for thin snapshots ?

Can't I just clone my filesystem and then create a thin pool ?


Sreyan Chakravarty
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