[linux-lvm] LVM2 Metadata structure, extents ordering, metadata corruptions

[Gionatan Danti]

Il 2022-09-27 12:10 Roberto Fastec ha scritto:
> questions
> 1. Given the premise 3. The corresponding LVM2 metadata/tables are and
> will be just a (allow me the term) "grid" "mapping that space" in an
> ordered sequence to in the subsequent use (and filling) of the RAID
> space "just mark" the used ones and the free ones? Or those grid cells
> will/could be in a messed order ?

Classical linear LVM volume (read: not lvmthin) are mostly concatenated 
4MB-sized chunk of space, but this is not a given (especially if some 
volumes changed in size).

> And explicitly I mean. In case of metadata corruption (always with
> respect of premise 3.) , could we just generate a dummy metadata table
> with all the extents marked as "used" in such a way that we can anyway
> access them

For linear volumes, one can try to setup a dmtable (or dummy metadata) 
to linearly read the data but, as stated above, this is far from 
reliable.

> 2. Does it exist a sort of "fsck" for the LVM2 metadata ? We do
> technical assistance and recently, specifically with those NAS devices
> that make use of LVM2, we have experienced really easy metadata
> corruption in occurence of just nothing or because of a electric power
> interruption (which is really astonishing). We mean no drives failures
> , no bad SMARTs . Just corruption from "nowhere" and "nocause"

For classical LVM, the metadata are actually backed up in ascii format 
unser /etc/lvm. While LVM itself keep a binary metadata representation, 
it also accept/store the textual so you can use the latter to restore 
your volumes.

Do you notice how I explicitly talked about *classical* volumes? This is 
because thin volumes (man lvmthin) use completely different, and much 
more complex, allocation strategies. Losing such metadata would kill the 
entire thin pool, and this is the reason a backup metadata volume is 
required for some operations. thincheck is effectively a sort ot 
"lvmthin fsck", but if you ever need to use it, be prepared to data loss 
(ranging from small to massive).

I saw various NAS that used custom-patched lvmthin volumes, and I 
suppose this is the root of your issues. If it is acceptable for your 
workload, try using classical LVM on these NAS.

Regards.

-- 
Danti Gionatan
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