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From: Sreyan Chakravarty <sreyan32@gmail.com>
To: LVM general discussion and development <linux-lvm@redhat.com>
Subject: Re: [linux-lvm] What is the use of thin snapshots if the external origin cannot be set to writable ?
Date: Tue, 24 Nov 2020 17:29:36 +0530	[thread overview]
Message-ID: <CAMaziXsh55b9VNgq40nVJwoFtD774_5i=JEiNZZOrdKpJsWQ_A@mail.gmail.com> (raw)
In-Reply-To: <20201123130412.GB2229389@localhost.localdomain>

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On Mon, Nov 23, 2020 at 6:34 PM Bryn M. Reeves <bmr@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 ?

>
>


-- 
Regards,
Sreyan Chakravarty

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  reply	other threads:[~2020-11-24 11:59 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-21  3:10 Sreyan Chakravarty
2020-11-23 12:44 ` Gionatan Danti
2020-11-23 13:04 ` Bryn M. Reeves
2020-11-24 11:59   ` Sreyan Chakravarty [this message]
2020-11-25  8:46     ` Sreyan Chakravarty
2020-11-25 12:38     ` Bryn M. Reeves
2020-11-25 15:31       ` Sreyan Chakravarty

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