[erlang-questions] Mnesia vs. ets+dets

Charles Hixson <>
Mon Nov 10 02:26:45 CET 2014

On 11/09/2014 12:56 PM, T Ty wrote:
> I usually advocate using Mnesia because any schema you come up with 
> you will later want some form of fault tolerance and as Joe Armstrong 
> keeps harping, you can't do fault tolerance with one node.
> That said, Mnesia does not automatically split data between tables. 
> You have to explicitly setup fragmented tables for it.
> To control how often Mnesia dumps from RAM to disc see:
> -mnesia dc_dump_limit Number. Controls how often disc_copies tables 
> are dumped from memory. Tables are dumped when filesize(Log) > 
> (filesize(Tab)/Dc_dump_limit). Lower values reduces cpu overhead but 
> increases disk space and startup times. The default is 4.
> On Sun, Nov 9, 2014 at 8:00 PM, Charles Hixson 
> < <mailto:>> wrote:
>     I'm going to be running on a single node, at least for the
>     foreseeable future, if that matters.
>     I expect the data to grow to be too large to fit into RAM, OTOH,
>     only a small subset will be needed at any one time. Also I expect
>     active items to have frequent updates, but most items to rarely be
>     updated (only when a purge is contemplated).  There doesn't seem
>     to be any way to control the frequency with which Mnesia
>     synchronizes to disk...but I also don't see any information about
>     how often it does so. Once every few minutes wouldn't be a
>     problem, but on every change would be prohibitive.  On the plus
>     side, Mnesia will automatically handle splitting the data between
>     tables (when the data grows too large to fit into one dets file).
>     One way of handling the problem is to operate out of an ets
>     database which pulls in data as needed from various dets files and
>     occasionally synchronizes with them.  This starts looking complex,
>     and Mnesia is clearly intended to solve problems *like* this.  I
>     just can't determine that it will handle *this* problem.  If I
>     could set the rate of synchronization between RAM and disk, it
>     would be the clear best approach...but I haven't even found any
>     documentation on that.
>     OTOH, I'm quite new to Erlang  (I've just written one successful
>     program) so I could easily be missing something that's extremely
>     obvious to someone more experienced.
>     Any comments or suggestions?
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