[erlang-questions] MNesia Distribution Questions

wde <>
Sat Oct 24 01:52:38 CEST 2009

Hello ,

I found  an old post ( 3 years ago) with this comment :

Two major problems with dets as of today (and of yeasterday)
1. 64 bit indexing - really easy to fix
2. repair time - this is a bit harder to fix, but one solution
could be along the following lines.
a) keep the index and the freelist in a different file,
b) have a file soley for the data. This file could be as easily
repaired as a disklog file. One could just chunk through it, one
term at a time, this way even large files will be fast to repair.
c) when an object is put on the freelist, it needs also to be

What about dets implementation ? 

do you consider that there are always the major problems of dets implementation ? 

In more general, concerning Mnesia (dets include), on which part of the code
we could work to improve this application ?

Thank you


======= le 22/10/2009, 08:15:15 vous écriviez: =======

>Ngoc Dao wrote:
>> Mnesia has 4 GB (or 2 GB?) limit per node. Is there a tutorial or doc
>> about how to create fragmented Mnesia DB, so that a very big DB can be
>> cut into pieces and saved on many nodes?
>Strictly speaking, Mnesia has no such limit.
>- The 32-bit VM can only address 4 GB of RAM
>- Normal linux kernels give the application 3 GB to use.
>   You can get past this using a largekernel.
>- Dets can address 2GB
>This means that for 32-bit Erlang:
>- The total amount of data residing in mnesia
>   ram_copies or disc_copies on a given node cannot
>   exceed 3GB or 4GB, depending on the OS.
>- The size of a disc_only copies table fragment* cannot
>   exceed 2GB.
>For 64-bit Erlang, the restriction on disc_only_copies
>remains, but RAM tables can be much, much bigger.
>However, most data types will take up twice as much space
>in 64-bit as in 32-bit. Binaries are one notable exception.
>* Tables can be fragmented using mnesia_frag. If not fragmented,
>the 'table fragment' will be the entire table.
>Ulf W
>Ulf Wiger
>CTO, Erlang Training & Consulting Ltd
>erlang-questions mailing list. See http://www.erlang.org/faq.html
>erlang-questions (at) erlang.org

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