[erlang-questions] running out of memory trying to add_frag on a large mnesia table

Chandru <>
Sun Nov 9 08:06:13 CET 2008


2008/11/7 Scott Lystig Fritchie <>

> Marc Sugiyama <> wrote:
>
> ms> I have a large, fragmented mnesia table.  I'm trying to add more
> ms> fragments to it (see below).
>
> I'm going to guess that you'll need to use the mnesia dump, filter, and
> restore.  Using the same hash function that mnesia_frag uses by default,
> your filter would spit out dump records with table names based on 70
> fragments instead of your current 10.  Er, and also replace the internal
> 'pvobject' state (inside the 'schema' table?) that contains the current
> fragmentation info with the new fragment number.  And perhaps a few
> other bits.
>
>
That is going to be very painful and error prone. I've tried it before and
it is not pretty. I would do it this way:

1. Take an mnesia backup
2. Delete the fragmented mnesia table from the schema
3. Create it again with the number of fragments you want
4. Traverse the backup and re-insert each record and let mnesia spread them
across fragments.

If the whole process of recreating the table takes too long, there are a
couple of ways to speed it up.

When doing (3), create the table as a ram_copies table
After (4), change the table type to disc_copies using
mnesia:change_table_copy_type/3

Chandru
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