Dne 31.7.2018 v 23:17 Marc MERLIN napsal(a):
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?
Sure thing, when there is no free space to extend your overprovisioned
thin-pool and you run out-of-space you hit the limit at some point....
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?
kernel metadata stored in _tmeta LV hold old mapping of all thin-volumes.
i.e. which thin-pool chunk belongs to which thin-volume.
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?
Yep - once you make ~16GiB metadata you can't make them any bigger (hard
internal limitation of existing thin-pool target implementation).
But you still need to remember you can run of space in your metadata if there
is heavy usage of many large thin volumes - so the value of free space should
be always somehow monitored...
Regards
Zdenek
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