[erlang-questions] Mnesia questions

[Ulf Wiger]

Chris Hicks wrote:
> I've looked all over to get the answers to these questions but I
> can't seem to find the answers I'm looking for, so here goes: Mnesia
> disk_copy tables to NOT use dets, but use a type of log file to
> handle the disk part of the storage. Does 32 bit Erlang still have a
> size limit with these files?

Disk log files are only limited by the 64-bit relative addressing.


 > What about 64 bit? The file size limit

It's the same, as far as I know. This goes for dets too.
The main difference is that disk_copies are also kept in RAM,
and this capacity is of course increased significantly. :)

> is per table anyway and I shouldn't have a problem with disc_copies
> if I make sure to break up my data set accordingly, correct? I keep
> reading there there will be a performance hit way before you reach
> the file size limit with dets anyway, does that hold true for
> disc_copies as well?

Disc_copies use the disk competely differently from dets.
They rely heavily on append, and basically streaming data to
and from disk. Disks generally do fairly well with this kind
of access pattern.


 > Anyone know at what table size the performance
> begins to degrade significantly?

It's a bit hardware-specific, but essentially, performance falls
off a cliff when your data set gets poor cache locality. When
this happens depends on your OS, disk subsystem etc., but once
you become limited by disk seek time, the performance is reasonably
predictable - roughly speaking, around 20 writes/sec.

SSD drives are much less cache-dependent, but I have no personal
experience using dets on SSD drives.

BR,
Ulf W
-- 
Ulf Wiger
CTO, Erlang Solutions Ltd, formerly Erlang Training & Consulting Ltd
http://www.erlang-solutions.com
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