[linux-lvm] How to check if thin volume or snapshot was ever activated

[linux-lvm] How to check if thin volume or snapshot was ever activated

[Gionatan Danti]

Dear all,
as you know, a thin snapshot does have the "k" (skip activation) flag 
set, so one has to force activation by ignoring the flag (or removing 
the flag itself).

I wonder: can we detect if a volume/snapshot was *ever* activated? My 
reasoning is that a never-activated snapshot surely did not receive any 
application writes, so it can be safely removed (ignoring snapshot 
retention policy).

Regards.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

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Re: [linux-lvm] How to check if thin volume or snapshot was ever activated

[Ming-Hung Tsai]

On Sat, Aug 7, 2021 at 7:43 PM Gionatan Danti <g.danti@assyoma.it> wrote:
>
> Dear all,
> as you know, a thin snapshot does have the "k" (skip activation) flag
> set, so one has to force activation by ignoring the flag (or removing
> the flag itself).
>
> I wonder: can we detect if a volume/snapshot was *ever* activated? My
> reasoning is that a never-activated snapshot surely did not receive any
> application writes, so it can be safely removed (ignoring snapshot
> retention policy).

It sounds like you intend to keep snapshots that have been updated
(written) since its creation, right?
The precise way might be checking the data mappings via thin_dump. An
updated device has data mappings with timestamps greater than the
device's creation time.

Checking device activation might not meet what you need. An activated
device might or might not receive any write, so you don't know whether
a device has been updated by looking into its activation history.

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Re: [linux-lvm] How to check if thin volume or snapshot was ever activated

[Ming-Hung Tsai]

On Mon, Aug 9, 2021 at 11:53 AM Ming-Hung Tsai <mingnus@gmail.com> wrote:
>
> On Sat, Aug 7, 2021 at 7:43 PM Gionatan Danti <g.danti@assyoma.it> wrote:
> >
> > I wonder: can we detect if a volume/snapshot was *ever* activated? My
> > reasoning is that a never-activated snapshot surely did not receive any
> > application writes, so it can be safely removed (ignoring snapshot
> > retention policy).
>
> It sounds like you intend to keep snapshots that have been updated
> (written) since its creation, right?
> The precise way might be checking the data mappings via thin_dump. An
> updated device has data mappings with timestamps greater than the

                ^^^^^^^^^^^^
> device's creation time.

sorry, I mean "greater than or equal to"

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Re: [linux-lvm] How to check if thin volume or snapshot was ever activated

[Gionatan Danti]

Il 2021-08-09 05:53 Ming-Hung Tsai ha scritto:
> It sounds like you intend to keep snapshots that have been updated
> (written) since its creation, right?

True.

> The precise way might be checking the data mappings via thin_dump. An
> updated device has data mappings with timestamps greater than the
> device's creation time.

Interesting approach, thanks.

> Checking device activation might not meet what you need. An activated
> device might or might not receive any write, so you don't know whether
> a device has been updated by looking into its activation history.

Sure, but it can be a reasonable approximation before further 
inspection. Also, I wonder if dumping data mappings can be slow for big 
volumes.

Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

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Re: [linux-lvm] How to check if thin volume or snapshot was ever activated

[Gionatan Danti]

Il 2021-08-10 10:40 Ming-Hung Tsai ha scritto:
> It depends on the number of thin volumes and the space utilization. A
> selective dump might also help, e.g.,
> # thin_dump <metadata> --dev-id <thin-id1> --dev-id <thin-id2> ...
> 
> The thin-ids could be obtained from lvs, using listing:
> # lvs -o lv_name,vg_name,thin_id
> or querying:
> # lvs -o thin_id --select <criteria> --noheadings

Greatly appreciated, thanks!

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

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