mnesia no disk write after restart

Michael McDaniel <>
Mon Apr 17 23:11:17 CEST 2006


 I changed the record name from ip to ipm with same behaviour.

 What I *did* find, though is that the following sequence causes
 the problem:

 1) start module as described below (in original post)

 2) enter records as described below (in original post)

 3) from command line run:
    erl -sname fu -config sys.config -s file script checkblock \
        -s erlang halt
 
    where checkblock contains:

    mnesia:start().
    timer:sleep(200).
    mnesia:wait_for_tables([ip,schema], 7000) ,
    X = block:dirty_sorted_list_table_localtime().
    Z = global:registered_names().
    io:format('~p~n~p entries~nregistered names: ~p~n', [X, length(X), Z]).
    mnesia:stop().

    and sys.config contents are described below (in original post)

 4) stop/restart module and no records get written to disk.  Actually,
    if I do step '3' above before the first records get written to disk
    then they never will (get written).

 The contents of block:dirty_sorted_list_table_localtime() is:

  dirty_sorted_list_table_localtime() ->
  io:format('*** LOCAL time ***~n',[]) ,
  lists:keysort(5, ?MODULE:dlt_localtime()) .

 and dlt_localtime() is:

  dlt_localtime() ->
  [ {element(1, X), element(2,X), element(3,X), element(4,X),
     calendar:universal_time_to_local_time(element(5,X)), element(6,X)}
     || X <- mnesia:dirty_match_object(ipm, {ipm, '_', '_', '_', '_', '_'}) 
  ] .
  
 

 So, it appears to be the heisenberg principal and the more I observerved,
 the less it worked.

 It appears that running step '3)' above causes mnesia to get confused
 about whether it should write to disk or ram; how can this be?

 Also, in original post I mentioned sending messages to module of what
 records to write.  That is one method of entering records.  During these
 tests, the module itself is reading the tail of a file and looking for
 patterns which triggers it to create a record and write to mnesia.  So,
 I am only entering text into a file to trigger record writing.  I
 mention this to eliminate conjecture that my sending process (there
 isn't one) is interfering with the module writing mnesia.  If anything,
 it appears that my observation (step '3)') causes a permanent problem
 with mnesia and/or my module function.

 Now, the question is "why is this happening?".  I need to be able to
 look at the db from an external program.

 Is there some other 'proper' way I should be checking the db contents?

thanks,

~Michael




On Mon, Apr 17, 2006 at 01:42:51AM -0700, Michael McDaniel wrote:
> BRIEF: module using mnesia works fine the first time I start it
>        (when the schema and table get created), and the db is
>        persistent for the next restart.  Henceforth, no records
>        write to disk, they only exist in memory, and further
>        restarts lose all but the records written on the initial
>        start (when the table was created).
> 
> 
> LONG:
> 
> My goal is a *persistent* database when restarting my module.
> 
> I am running R10B-10 on Linux.
> 
> I have abbreviated the following code.  I think all the pertinent
> bits are there.
> 
> I start my module with the following fun:
> 
> 
> start() -> 
> 
>  erlang:set_cookie( node(), some_cookie )
> 
>  mnesia:create_schema([node()]) ,
>  mnesia:start() ,
> 
> 
>  case catch mnesia:table_info(?DB, attributes) of
> 
>     L when is_list(L) -> 
>                         ok ;
> 
>     _                 ->
>        mnesia:create_table(?DB ,
>           [ {disc_copies, [node()]} ,
>             {attributes, record_info(fields, ?DB)} ,
>             {type, set}
>           ])
> 
>  end,
>     
>  mnesia:wait_for_tables([?DB,schema], 21000)
> .
> 
> 
> the write function is:
> 
> write(Rec) ->
>     Trans = fun() -> mnesia:write(Rec) end ,
>     mnesia:transaction(Trans) ,
>     os:cmd("/bin/sync")
> .
> 
> 
> I run a loop that waits for messages of records to write.
> 
> I start the module from a script containing:
> 
> 
> #!/bin/sh
> 
> HOST=`/bin/hostname`
> 
> /bin/su mmcdanie -c "cd /home/erl ; /usr/local/bin/erl              \
>                  -sname  ${HOST}_block -config /home/erl/sys.config \
>                  -setcookie xus -env TERM linux -env HOME /home/erl \
>                  -home /home/erl -heart -detached                   \
>                  -s block start /var/log/auth"
> #% end
> 
> 
> and the contents of sys.config:
> 
> [{mnesia, [{dir, "/home/erl/Mnesia.block"}  ,
> %%           {debug, trace}                 ,
>            {schema_location, disc}]}].
> 
> 
> I start the application the first time with no Mnesia.block
> directory created.  Module starts fine, directory and db files
> are created, and I can send messages which get written to
> the database.  I see the .LOG file change and the record.DCL
> file get created.
> 
> I stop the module and restart it and I can see the records
> in the database.
> 
> I send more messages and the records get written though only
> to memory.  A new record.DCL file never gets created.  The
> .LOG file(s) never change.  'Never' is waiting for over 10
> minutes (and {dump_log_time_threshold,180000}).
> 
> I run mnesia:system_info from another node:
> 
> ()2> mnesia:system_info().
> ===> System info in version "4.2.5", debug level = none <===
> disc. Directory "/home/erl/Mnesia.block" is used.
> use fallback at restart = false
> running db nodes   = []
> stopped db nodes   = []
> master node tables = []
> remote             = [ip]
> ram_copies         = [schema]
> disc_copies        = []
> disc_only_copies   = []
> [{,ram_copies},{,disc_copies}] = [schema]
> [{,disc_copies}] = [ip]
> 3 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: []
> yes
> 
> 
> The '0 logged to disc' is after having written a new record that
> does show in the database.
> 
> 
> QUESTIONS:
> 
> 1) Does anyone see something obvious I am overlooking here?
> 
> 2) Any suggestions on what to do differently?
> 
> 3) What is the next bit of information anyone needs to help?
> 
> 
> thanks,
> 
> ~Michael
> P.S. I am doing essentially the same thing in another module
> that I start from yaws and it works fine (i.e. persistent db
> between restarts).  I have not sleuthed out the cause of the
> different behaviours!  In the other module (that works) I do
> not even have the case statement protecting the create_table/2
> fun.

-- 
Michael McDaniel
Portland, Oregon, USA
http://autosys.us
+1 503 283 5284



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