CODEWITHSUNDEEP
erlangelixirmnesia
lapinkoira
lapinkoira

Reputation: 8978

Size in MB of mnesia table

How do you read the :mnesia.info?

For example I only have one table, some_table, and :mnesia.info returns me this.

---> Processes holding locks <--- 
---> Processes waiting for locks <--- 
---> Participant transactions <--- 
---> Coordinator transactions <---
---> Uncertain transactions <--- 
---> Active tables <--- 
some_table: with 16020    records occupying 433455   words of mem
schema         : with 2        records occupying 536      words of mem
===> System info in version "4.15.5", debug level = none <===
opt_disc. Directory "/home/ubuntu/project/Mnesia.nonode@nohost" is NOT used.
use fallback at restart = false
running db nodes   = [nonode@nohost]
stopped db nodes   = [] 
master node tables = []
remote             = []
ram_copies         = ['some_table',schema]
disc_copies        = []
disc_only_copies   = []
[{nonode@nohost,ram_copies}] = [schema,'some_table']
488017 transactions committed, 0 aborted, 0 restarted, 0 logged to disc
0 held locks, 0 in queue; 0 local transactions, 0 remote
0 transactions waits for other nodes: []

Also calling:

:mnesia.table_info("some_table", :size)

It returns me 16020 which I think is the number of keys, but how can I get the memory usage?

Upvotes: 2

Views: 1087

Answers (2)

codeadict
codeadict

Reputation: 2753

First, you need mnesia:table_info(Table, memory) to obtain the number of words occupied by your table, in your example you are getting the number of items in the table, not the memory. To transform that value to MB, you can first use erlang:system_info(wordsize) to get the word size in bytes for your machine architecture(on a 32 bit system a word is 4 bytes and 64 bits it's 8 bytes), multiply it by your Mnesia table memory to obtain the size in bytes and finally transform the value to MegaBytes like: 

MnesiaMemoryMB = (mnesia:table_info("some_table", memory) * erlang:system_info(wordsize)) / (1024*1024).

Upvotes: 6

Roman Rabinovich
Roman Rabinovich

Reputation: 918

You can use erlang:system_info(wordsize) to get the word size in bytes, on a 32 bit system a word is 32 bits or 4 bytes, on 64 bit it's 8 bytes. So your table is using 433455 x wordsize.

Upvotes: 2

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