[linux-lvm] Why use thin_pool_autoextend_threshold < 100 ?

[Marc MERLIN]

On Tue, Jul 31, 2018 at 02:35:42PM +0200, Zdenek Kabelac wrote:
> If you monitor amount of free space for data AND for metadata in thin-pool
> yourself you can keep easily threshold == 100.
 
Understood. Two things:
1) basically threshold < 100 allows you to hit the limit, have LVM pause
IO, allocate more blocks, and resize the filesystem for you.
However, if you're not monitoring this, it's ultimately just the same as
having threshold = 100 and hoping that you won't hit the limit, except
that you're adding the complexity of resizes in the mix. Correct?

2) I wasn't quite clear on what metadata was used for, and I let
vgcreate pick a default amount for me. Am I correct that it basically
tracks block usage and maybe LVM snapshots that I'm not going to use,
and that therefore if I don't resize my LV, I don't really have to
worry about metadata running out?

> Just don't forget when you upsize 'data' - you should also typically
> extend also metadata -  it's not uncommon issue user  start with small
> 'data' & 'metadata' LV with thin-pool - then  continue to only extend
> thin-pool 'data' volume and ignore/forget about metadata completely
> and hit the full metadata device - which can lead to many troubles
> (hitting full dataLV is normally not a big deal).

Thanks for the warning. Given that I started with the maximum size and
don't plain on ever extending (to be fair, I can't), I should be ok
there, correct?

Thanks,
Marc
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