Fragment not locked

[wojtek...@me.com]

I have a fragmented table in Mnesia. When I try to add a new fragment, as described in documentation:

 

mnesia:change_table_frag(name, {add_frag, [node()]}).

 

I am getting an error

 

{aborted,{"add_frag: Fragment not locked",5}} 

 

What could be the cause of this? What can I do to add a new fragment?

 

Thanks,

Wojtek

[Stanislav Ledenev]

Are you calling "change_table_frag" within a transaction (i.e. mnesia:activity)?

вс, 11 июл. 2021 г. в 16:57, <wojtek...@me.com>:

[wojtek...@me.com]

> Are you calling "change_table_frag" within a transaction (i.e. mnesia:activity)?

No I do not. Should I? I followed examples in documentation at
http://erlang.org/doc/apps/mnesia/Mnesia_chap5.html#table-fragmentation
where activity is used for other operations, but not for add_frag.

Thanks,
Wojtek

[Stanislav Ledenev]

Following the documentation, are you trying to reproduce example for {hash_state, Term}? 

The line "mnesia:change_table_frag(prim_dict, {add_frag, [node()]})."  can be found only there.

Or you've somehow changed those examples for your own needs? If so please provide a full list of your steps.

пн, 12 июл. 2021 г. в 14:36, <wojtek...@me.com>:

[Wojtek Surowka]

> Following the documentation, are you trying to reproduce example for
> {hash_state, Term}? The line "mnesia:change_table_frag(prim_dict, {add_frag, [node()]})." can be found only there.
> Or you've somehow changed those examples for your own needs? If so please provide a full list of your steps.

I do not try to reproduce this example. I created table using {atomic, ok} = mnesia:create_table(tablename, [{attributes, record_info(fields, tablename)}, {disc_only_copies, [node()]}])

I write to this table with
ok = mnesia:activity(sync_transaction, fun () -> mnesia:write(RecordToWrite) end, [], mnesia_frag) this is called within mnesia:transaction because also other tables are accessed. A write like this happens several times every minute, and nearly always record is replacing an already existing one with the same key.

I read from this table with
mnesia:activity(sync_dirty, fun () -> mnesia:read(tablename, RecordSpec) end, [], mnesia_frag) Reading happens occasionally, when end users activity triggers it.

When I detect that fragment tables are bigger than some threshold, I add a new fragment with mnesia:change_table_frag(tablename, {add_frag, [node()]}) I used [node()] as the second element of add_frag tuple since documentation says it is either the list of nodes or a frag_dist result. I use one node, so [node()] seemed appropriate.

It was all working for some time, but since some time it stopped - to be precise, reads and writes work correctly, but adding a new fragment fails every time. Right now the table has 16 fragments, total of 1.7M records, and the biggest fragment is now 1.6GB on disk.

I considered copying all data, recreating the table with more fragments and copying back, but I hope there is something I can do to avoid this procedure.

(Sorry for posting this by mistake to you instead of the mailing list)
Thanks,
Wojtek

[Stanislav Ledenev]

I've looked at the source code of Mnesia and it has this comment in mnesia_frag.erl:
   %% Tried to move record to fragment that not is locked
   mnesia:abort({"add_frag: Fragment not locked", NewN})

But before this step all required locks are made.
Unfortunately I can't help you with this. Sorry. I hope  someone more familiar with Mnesia's code will help you.

пн, 12 июл. 2021 г. в 15:29, Wojtek Surowka <wojtek...@me.com>:

[Wojtek Surowka]

>I've looked at the source code of Mnesia and it has this comment in mnesia_frag.erl:
> %% Tried to move record to fragment that not is locked
> mnesia:abort({"add_frag: Fragment not locked", NewN})
> But before this step all required locks are made.
> Unfortunately I can't help you with this. Sorry. I hope someone more familiar with Mnesia's code will help you.

Thank you for trying! I always have the nuclear option of copy/delete/recreate with more fragments/copy back...

Thanks,
Wojtek