r19据说支持了mnesia_ext，终于可以给那个恶心2gb limit的mnesia换存储引擎了
先下载r19源码，编译
./otp_build all -a ~/dev/erlang/r19
./Install ~/dev/erlang/r19
关于mnesia_ext，r19说是有了，但是怎么做，没有文档，也没有资料，最后在otp的pull request里面找
https://github.com/erlang/otp/pull/858/files
看到create_schema，发现有两个参数了，看来似乎是有了，但是怎么写呢
再次google mnesia_ext，找到下面这个
https://github.com/klarna/mnesia_pg
搜索
grep -nr "mnesia:create_schema" ./
看到
mnesia:create_schema([node()], [{backend_types, [{pg, mnesia_pg}]}]),然后去mnesia_pg里面看了下，貌似有个
-behaviour(mnesia_backend_type).
那我们直接在r19里面搜索
grep -nr "module(mnesia_backend_type)" ./
ok，找到了
mnesia_backend_type.erl
%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 1996-2015. All Rights Reserved.
%%
%% Licensed under the Apache License, Version 2.0 (the "License");
%% you may not use this file except in compliance with the License.
%% You may obtain a copy of the License at
%%
%% http://www.apache.org/licenses/LICENSE-2.0
%%
%% Unless required by applicable law or agreed to in writing, software
%% distributed under the License is distributed on an "AS IS" BASIS,
%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
%% See the License for the specific language governing permissions and
%% limitations under the License.
%%
%% %CopyrightEnd%
%% %%
%% Behaviour definition for mnesia backend types.
%% %%%header_doc_include -module(mnesia_backend_type). -export([behaviour_info/1]). %%%header_doc_include %%%impl_doc_include %% Note that mnesia considers all callbacks mandatory!!
%%
behaviour_info(callbacks) ->
[
{add_aliases, 1}, % (Aliases) -> ok
{check_definition, 4}, % (TypeAlias, Tab, Nodes, Properties) -> ok
{close_table, 2}, % (TypeAlias, Tab) -> ok
{create_table, 3}, % (TypeAlias, Tab, Properties) -> ok
{delete, 3}, % (TypeAlias, Tab, Key) -> true | ok
{delete_table, 2}, % (TypeAlias, Tab) -> ok
{first, 2}, % (TypeAlias, Tab) -> Key::Term | '$end_of_table'
{fixtable, 3}, % (TypeAlias, Tab, Bool) -> ok | true
{last, 2}, % (TypeAlias, Tab) -> Key::Term | '$end_of_table'
{index_is_consistent,3}, % (TypeAlias, IxTag, Bool) -> ok
{init_backend, 0}, % () -> ok
{info, 3}, % (TypeAlias, Tab, Item) -> Term
{insert, 3}, % (TypeAlias, Tab, Object) -> ok
{lookup, 3}, % (TypeAlias, Tab, Key) -> [Objects]
{is_index_consistent,2}, % (TypeAlias, IxTag) -> Bool
{load_table, 4}, % (TypeAlias, Tab, Reason, CsList) -> ok
{match_delete, 3}, % (TypeAlias, Tab, Pattern) -> ok
{next, 3}, % (TypeAlias, Tab, Key) -> Key::Term | '$end_of_table'
{prev, 3}, % (TypeAlias, Tab, Key) -> Key::Term | '$end_of_table'
{receiver_first_message, 4}, % (Sender, FirstMsg, Alias, Tab) -> {Size, State}
{receive_data, 5}, % (Data, Alias, Name, Sender, State) -> {more, State} | {{more, Msg}, State}
{receive_done, 4}, % (Alias, Tab, Sender, State) -> ok
{real_suffixes, 0}, % () -> [FileSuffix]
{remove_aliases, 1}, % (Aliases) -> ok
{repair_continuation, 2}, % (Continuation, MatchSpec) -> Continuation
{select, 1}, % (Continuation) -> {[Match], Continuation'} | '$end_of_table'
{select, 3}, % (TypeAlias, Tab, Pattern) -> {[Match], Continuation'} | '$end_of_table'
{select, 4}, % (TypeAlias, Tab, MatchSpec, Limit) {[Match], Continuation'} | '$end_of_table'
{sender_init, 4}, % (TypeAlias, Tab, LoadReason, Pid) ->
% {standard, Init(), Chunk()} | {Init(), Chunk()}
{semantics, 2}, % (TypeAlias, storage | types | index_fun | index_types) ->
% ram_copies | disc_copies, set | ordered_set | bag, fun(), ordered | bag
{slot, 3}, % (TypeAlias, Tab, Pos) -> '$end_of_table' | Objects | {error, Reason}
{sync_close_table, 2}, % (TypeAlias, Tab) -> ok
{tmp_suffixes, 0}, % () -> [FileSuffix]
{update_counter, 4}, % (TypeAlias, Tab, Counter, Val) -> NewVal
{validate_key, 6}, % (TypeAlias, Tab, RecName, Arity, Type, Key) -> {RecName, Arity, Type}
{validate_record, 6} % (TypeAlias, Tab, RecName, Arity, Type, Obj) -> {RecName, Arity, Type}
]. %%%impl_doc_include %% -type tab() :: atom().
%% -type alias() :: atom().
%% -type rec_name() :: atom().
%% -type type() :: set | bag | ordered_set.
%% -type proplist() :: [{atom(), any()}].
%% -type key() :: any().
%% -type db_object() :: tuple(). %% -type matchspec() :: ets:match_spec().
%% -type limit() :: integer() | infinity. %% -type cont_fun() :: any().
%% -type cont() :: '$end_of_table' | cont_fun(). %% -callback check_definition(alias(), tab(), [node()], proplist()) -> ok.
%% -callback create_table(alias(), tab(), proplist()) -> tab().
%% -callback load_table(alias(), tab(), any()) -> ok.
%% -callback delete_table(alias(), tab()) -> ok.
%% -callback first(alias(), tab()) -> key().
%% -callback last(alias(), tab()) -> key().
%% -callback next(alias(), tab(), key()) -> key().
%% -callback prev(alias(), tab(), key()) -> key().
%% -callback insert(alias(), tab(), db_object()) -> ok.
%% -callback lookup(alias(), tab(), key()) -> [db_object()].
%% -callback delete(alias(), tab(), key()) -> ok.
%% -callback update_counter(alias(), tab(), key(), integer()) -> integer().
%% -callback select(cont()) -> {list(), cont()}.
%% -callback select(alias(), tab(), matchspec()) -> list() | '$end_of_table'.
%% -callback select(alias(), tab(), matchspec(), limit()) -> {list(), cont()}.
%% -callback slot(alias(), tab(), integer()) -> key().
%% -callback validate_key(alias(), tab(), rec_name(), arity(), type(), key()) ->
%% {rec_name(), arity(), type()}.
%% -callback validate_record(alias(),tab(),rec_name(),arity(),type(),db_obj()) ->
%% {rec_name(), arity(), type()}.
%% -callback repair_continuation(cont(), matchspec()) -> cont().
应该就是这些了吧，具体的和后端存储引擎的对接，还是等大牛吧。