Re: [linux-lvm] thin: pool target too small

[Duncan Townsend <duncancmt@gmail.com>]

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On Tue, Sep 29, 2020, 10:54 AM Zdenek Kabelac <zkabelac@redhat.com> wrote:

> Dne 29. 09. 20 v 16:33 Duncan Townsend napsal(a):
> > On Sat, Sep 26, 2020, 8:30 AM Duncan Townsend <duncancmt@gmail.com
> > <mailto:duncancmt@gmail.com>> wrote:
> >
> >>      > > There were further error messages as further snapshots were
> attempted,
> >      > > but I was unable to capture them as my system went down. Upon
> reboot,
> >      > > the "transaction_id" message that I referred to in my previous
> message
> >      > > was repeated (but with increased transaction IDs).
> >      >
> >      > For better fix it would need to be better understood what has
> happened
> >      > in parallel while 'lvm' inside dmeventd was resizing pool data.
> >
>
> So the lvm2 has been fixed upstream to report more educative messages to
> the user - although it still does require some experience in managing
> thin-pool kernel metadata and lvm2 metadata.
>

That's good news! However, I believe I lack the requisite experience. Is
there some documentation that I ought to read as a starting point? Or is it
best to just read the source?

>     To the best of my knowledge, no other LVM operations were in flight at
> >     the time. The script that I use issues LVM commands strictly
>
> In your case - dmeventd did 'unlocked' resize - while other command
> was taking a snapshot - and it happened the sequence with 'snapshot' has
> won - so until the reload of thin-pool - lvm2 has not spotted difference.
> (which is simply a bad race cause due to badly working locking on your
> system)
>

After reading more about lvm locking, it looks like the original issue
might have been that the locking directory lives on a lv instead of on a
non-lvm-managed block device. (Although, the locking directory is on a
different vg on a different pv from the one that had the error.)

Is there a way to make dmeventd (or any other lvm program) abort if this
locking fails? Should I switch to using a clustered locking daemon (even
though I have only the single, non-virtualized host)?

>     Would it be reasonable to use vgcfgrestore again on the
> >     manually-repaired metadata I used before? I'm not entirely sure what
>
> You will need to vgcfgrestore - but I think you've misused my passed
> recoverd
> piece, where I've specifically asked to only replace specific segments of
> resized thin-pool within your latest VG metadata - since those likely have
> all the proper mappings to thin LVs.
>

All I did was use vgcfgrestore to apply the metadata file attached to your
previous private email. I had to edit the transaction number, as I noted
previously. That was a single line change. Was that the wrong thing to do?
I lack the experience with lvm/thin metadata, so I am flying a bit blind
here. I apologize if I've made things worse.

While you have taken the metadata from 'resize' moment - you've lost all
> the thinLV lvm2 metadata for later created one.
>
> I'll try to make one for you.
>

Thank you very much. I am extremely grateful that you've helped me so much
in repairing my system.

>     to look for while editing the XML from thin_dump, and I would very
> >     much like to avoid causing further damage to my system. (Also, FWIW,
> >     thin_dump appears to segfault when run with musl-libc instead of
>
> Well - lvm2 is glibc oriented project - so users of those 'esoteric'
> distribution need to be expert on its own.
>
> If you can provide coredump or even better patch for crash - we might
> replace the code with something better usable - but there is zero testing
> with anything else then glibc...
>

Noted. I believe I'll be switching to glibc because there are a number of
other packages that are broken for this distro.

If you have an interest, this is the issue I've opened with my distro about
the crash: https://github.com/void-linux/void-packages/issues/25125 . I
despair that this will receive much attention, given that not even gdb
works properly.

Thanks again!
--Duncan Townsend

P.S. This was written on mobile. Please forgive my typos.

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