[erlang-questions] Mnesia and the 4Gb Limit

Ulf Wiger (TN/EAB) <>
Fri Feb 15 11:48:22 CET 2008


Dan Gudmundsson skrev:
> Only tables with storage 'disc_only_copies' uses dets,
 > disc_copies uses disk_log to store the data.
> 
> /Dan


Looking back at this experiment:
http://www.erlang.org/pipermail/erlang-questions/2005-November/017728.html

I can't recall whether I actually put more than 10 GB into one
single disc_copy table. From the text, it appears as if it was
more than 10 GB in RAM, but less than 4G on disk. Still, it gives
an idea of the kind of characteristics you can get.

BR,
Ulf W

> 
> Chandru wrote:
>> On 14/02/2008, David Lloyd <> wrote:
>>>  Hi There,
>>>
>>>  Does Mnesia/Dets still have the 4GB limit? Answers I've found on the
>>>  'net seem to indicate that 64bit Erlang has, essentially, erased this limit.
>>>
>>>  I'm looking at a database table that miht be larger than 4Gb; it's
>>>  already an in memory, MySQL table (MySQL cluster uses an in memory table
>>>  structure) so I don't seem to have a "lack" of memory [last time I
>>>  looked the min. memory in any one node on this cluster is 8Gb).
>>>
>> >From reading the manuals, it looks like dets still has this limit.
>> What 64 bit erlang does is remove the restriction that an erlang node
>> can only address 4GB of memory. This basically means you can have a
>> database of more than 4GB, but *each* mnesia table on disk cannot be
>> more than 2GB. You can use table fragmentation to get around this. I
>> guess this restriction won't apply to mnesia ram_copies tables.
>>
>> I have created databases of up to 15GB without any problem using 64
>> bit erlang, but they were fragmented disc_copies tables.
>>
>> cheers
>> Chandru
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