[erlang-questions] Unable to start mnesia DB.

Karolis Petrauskas <>
Tue Apr 15 06:57:47 CEST 2014


Hi,

Recently this problem reappeared: my mnesia DB started to load very
slowly. The main problem was single disc_copies table with ~500k small
records. This time it was loading for 35 minutes. This table is of
type "bag" and has one secondary index.

Based on previous emails, I assumed that the problems are caused by
the index. I dropped the index, altrough observed no notable
improvement (in startup time). Then I changed table to
disc_only_copies. The DB started to load fast (<1min) event with
secondary index (I have recreated it).

Another strange thing here is that the mnesia:foldl/3 for this table
was very slow. I was just copying several fields from this table to
another table. I have aborted this operation after waiting for several
hours. The foldl operation completed in ~2 minutes after i changed the
source table type from disc_copies to disc_only_copies.

Currently I think the problem is related to uniqueness of the key in
the table. For some time the application worked so, that each key had
not more than 100 records per key (the table is of type "bag").
Recently the application started to generate more records (1k or event
10k) per key.

Is it normal/expected behaviour of Mnesia, or should I try to create
scripts for reproducing this problem and report it as a bug?

The application is running on Erlang R16B02 on Debian OS.

Karolis

On Sun, Jul 14, 2013 at 10:59 PM, Karolis Petrauskas
<> wrote:
> Hi,
>
>     Thanks for your response. I have 1 secondary index in the entire
> DB. I will try to load the DB with the index dropped.
>
> Karolis
>
> On Fri, Jul 12, 2013 at 10:12 AM, Ignas Vyšniauskas <> wrote:
>> On 07/09/2013 07:16 PM, Karolis Petrauskas wrote:
>>> After the DB become available, I stopped the node by invoking
>>> init:stop(). Then I repeated the test. It is still running, but
>>> timings look the same (2 cores were busy for 2.5 hours, and after
>>> that 1 core is loaded fully, expected for ~5.5 hours). For the second
>>> start up attempt the DB has no DCL files, only DCD are left after
>>> first load.
>>
>> A poke in the dark: do you maybe have lots of secondary indices? These
>> will not be stored on disk and so will be recreated each time the tables
>> are loaded, I've experienced very long mnesia startup times in such
>> cases. Otherwise -- no idea.
>>
>> --
>> Ignas



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