[erlang-questions] Mnesia and schema locks

[Dan Gudmundsson]

Well you will need to figure out what ,<6502.2299.18> <6502.2302.18> are
doing, but they probably waiting
for other locks which are occupied by the busy processes you wrote about.
But you will have to look at that, debugging mnesia is just following the
breadcrumbs around the system.

mnesia_locker:get_held_locks() and mnesia_locker:get_lock_queue() may also
help.

Using observer to attach to the different nodes is probably easiest, then
you can get a stacktrace of each process,
normally when I do it I don't have a live system. If I want to debug post
mortem I use mnesia_lib:dist_coredump()
to collect each mnesia nodes state and analyse them. Though with many nodes
it will take some time to debug or
figure out why it appears to be hanging.


On Wed, Feb 14, 2018 at 6:39 PM Loïc Hoguin <essen@REDACTED> wrote:

> Hello,
>
> We are trying to debug an issue where we observe a lot of contention
> when a RabbitMQ node go down. It has a number of symptoms and we are in
> the middle of figuring things out.
>
> One particular symptom occurs on the node that restarts, it gets stuck
> and there are two Mnesia locks:
>
> [{{schema,rabbit_durable_route},read,{tid,879886,<6502.2299.18>}},
>   {{schema,rabbit_exchange},read,{tid,879887,<6502.2302.18>}}]
>
> The locks are only cleared when the other node in the cluster stops
> being so busy deleting data from a number of tables (another symptom)
> and things go back to normal.
>
> Part of the problem is that while this is going on, the restarting node
> cannot be used, so I would like to understand what conditions can result
> in these locks staying up for so long. Any tips appreciated!
>
> Thanks in advance,
>
> --
> Loïc Hoguin
> https://ninenines.eu
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> erlang-questions mailing list
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>
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