[erlang-bugs] All keys in the same slot (Mnesia/dets)

[Igor Ribeiro Sucupira]

Hum... mnesia_frag_hash and dets both use phash2, so it makes sense
that the keys are poorly distributed among the slots of each dets
table that is a disc_only_copies fragment.  :-(
I guess we'll have to deal with that somehow.

However, I still don't understand why one fragment has 1536 slots and
the other has 524288, in the example below.

Thank you.
Igor.

On Tue, Jun 7, 2011 at 3:19 AM, Igor Ribeiro Sucupira <igorrs@REDACTED> wrote:
> Hello.
>
> I was running a pool with Erlang/OTP R13B04 until the beginning of the
> last week, when we upgraded to R14B02.
>
> The reason we upgraded is that everyday we were experiencing a lot of
> corruption in Mnesia's fragments (disc_only_copies). We had Erlang
> processes checking the fragments periodically and, in case of
> problems, we would delete the fragment (mnesia:del_table_copy) and
> clone it again from its replica.
> Given that Erlang/OTP R14B01 fixed a lot of concurrency issues with
> dets, I believe those bugs were affecting us because we perform a lot
> of dirty reads, what must cause more concurrent operations in dets
> tables.
>
> Anyway, since the upgrade to R14B02, no corrupted fragment has been
> detected.  :-)
>
> But then I observed that, after the upgrade, most of the servers are
> performing much better (spending less time in I/O operations), while
> some of them almost didn't change in that respect.
>
> Taking a look at Mnesia's directory in each server, I noticed that the
> fragment files in one table (uc) are smaller in the servers that are
> performing better.
>
> Example of a fragment in a "slower" server:
> 59M     uc_frag1004.DAT
>
> Example of a fragment in a "faster" server:
> 34M     uc_frag598.DAT
>
> Since uc is a bag, I thought it could be because uc_frag598, for
> example, has less records than uc_frag1004. But I copied both to my
> box and saw I was wrong:
>
> 1> {ok, F1004} = dets:open_file("uc_frag1004.DAT").
> {ok,#Ref<0.0.0.33>}
> 2> {ok, F598} = dets:open_file("uc_frag598.DAT").
> {ok,#Ref<0.0.0.41>}
> 3> dets:info(F1004, no_objects).
> 280105
> 4> dets:info(F598, no_objects).
> 303074
> 5> dets:info(F1004, no_keys).
> 1404
> 6> dets:info(F598, no_keys).
> 1476
>
> The only (very) relevant difference I found between them was in the slots:
>
> 7> dets:info(F1004, no_slots).
> {256,1536,2097152}
> 8> dets:info(F598, no_slots).
> {524288,524288,33554432}
>
> The weirdest difference being that all records in uc_frag1004 are in
> the same slot!
>
> 9> length([S || S <- lists:seq(0, element(2, dets:info(F1004,
> no_slots)) - 1), length(dets:slot(F1004, S)) > 0]).
> 1
> 10> length([S || S <- lists:seq(0, element(2, dets:info(F598,
> no_slots)) - 1), length(dets:slot(F598, S)) > 0]).
> 489
>
>
> So... what may have happened and what can I do to fix it?
>
> Thank you.
> Igor.
>
> --
> "The secret of joy in work is contained in one word - excellence. To
> know how to do something well is to enjoy it." - Pearl S. Buck.