Discourse - DeviceMapper causing corruption?

[Chris Murphy]

On Mon, Mar 21, 2016 at 1:03 PM, Philip Rhoades <phil at pricom.com.au> wrote:
> People,
>
> I had a couple of issues to sort out with installing the Docker Discourse
> app and while that was being done people made these comments:
>
> "Devicemapper is non starter, fails spectacularly under load and causes
> corruption. We block setup if we detect devicemapper. You need aufs or
> another better supported docker filesystem."
>
> - which was not true - it did install without resorting to aufs.
>
> also:
>
> "Redhat team get very upset when we mention that it just does not work for
> us, but release after release they say there are no bugs left, and each time
> we keep seeing Discourse users complain about corruption due to device
> mapper."
>
> Any comments?

On the one hand, I'd say, talk is cheap, show me the bug.

On the other hand, having blown up thinly provisioned pools quite a
few times early on, I basically couldn't handle how non-deterministic
the blow ups were, and how difficult the LVM and devicemapper tools
are to understand the state of the pool, and fix it. So I gave up and
went down the Btrfs path just because I understand it better, and
there are only so many rabbit holes one has time and interest to go
down.

For a good bug report you either need a way to debug it, i.e. you need
familiarity with that thing you are debugging. Or you need a concise
set of reproduce steps (and a list of versions) so someone else, like
a dev or more experienced user, can reproduce that environment and
steps and debug it. Without one of those two things at least, I think
it's unreasonable to say "hi this is busted" and then just walk away.

aufs is just not workable in my opinion because it's not in the
mainline kernel. So any sense of portability can't be that important
if you're going to depend on aufs. There are plenty of bugs to go
around at this point, between overlay(fs), btrfs, and devicemapper
although I think by now most of the devicemapper stuff is probably the
more stable of the three (?) just a guess really. I had 500+ container
states as Btrfs subvolumes and never ran into a Btrfs related problem
though. What are the CoreOS folks using? Overlay or dm?

-- 
Chris Murphy