maria-developers team mailing list archive

[Sergei Golubchik <sergii@xxxxxxxxx>]

Hi, Paul!

On Dec 01, Paul McCullagh wrote:
>
> Thanks for the heads-up on that!
>
> The code in question is as follows:
>
> #ifndef WILL_BE_DELETED_LATER
>   /*
>     for now, only InnoDB supports 2pc. It means we can always safely
>     rollback all pending transactions, without risking inconsistent data
>   */
>   DBUG_ASSERT(total_ha_2pc == (ulong) opt_bin_log+1); // only InnoDB and binlog
>   tc_heuristic_recover= TC_HEURISTIC_RECOVER_ROLLBACK; // forcing ROLLBACK
>   info.dry_run=FALSE;
> #endif
>
> What would be the affect of removing these lines?

I'd better start from "what is the effect of these lines" :)

2PC (with a binlog) works as follows:

   prepare in all affected storage engines
   binlog write
   commit in all affected storage engines

XA recovery (with a binlog) works as follows:

   ask storage engines about the prepared but not committed transactions
   ask engines to commit every such a transaction that is in a binlog
   and ask engines to rollback other prepared transactions

Now, if a binlog doesn't indicate a crash (last binlog file was closed
properly, showing a normal shutdown procedure) or, for example, there're
no binlog files at all, the server cannot perform the recovery - it
doesn't know what transactions should be committed and what transactions
should be rolled back. A transaction may be prepared in one engine and
already committed (or rolled back) in another. In this case the server
requests the user to make the decision, the user has to request an
explicit commit or rollback with --tc-heuristic-recover command-line
switch.

But if there's only one XA-capable storage engine, it's always safe to
rollback all prepared transactions, there can be no other engine that
has them committed. That's what the ifdef-ed code does - if InnoDB is
the only XA-capable storage engine on recovery without binlog it forces
a rollback of all not committed transactions, preserving the pre-XA
InnoDB behavior.

> And what do you think we would need to test when we take out the lines?

Start a transaction, do some table writes. Crash the server during the
2pc, for example:

  SET @@debug="d,crash_commit_after_prepare";
  COMMIT;

delete all binlog files (or only the last one), try to start mysqld - it
should abort with an error. The error message tells what to do - I hope
you'll find it clear enough (but please complain, if not).

> On Dec 1, 2009, at 1:14 PM, Sergei Golubchik wrote:
>> On Dec 01, Paul McCullagh wrote:
>>>
>>> This patch changes MariaDB code in one case:
>>>
>>> File handler.cc, Line 1591
>>>
>>> DBUG_ASSERT(total_ha_2pc == (ulong) opt_bin_log+1); // only InnoDB and 
>>> binlog
>>>
>>> This line has been commented out, because the assertions fails now
>>> that PBXT supports XA.
>>>
>>> Although this assertion implies that not everything will work, I have
>>> not found any problem with the stability of MySQL due to the addition
>>> of PBXT XA.
>>
>> Eh, no, not exactly. This assertion was to make sure that the ifdef'ed
>> code only runs when InnoDB is the only XA-capable engine.
>>
>> As it's no longer the case, you need to remove the whole ifdef block,
>> not only the assert.
>>
>>> PBXT XA can be disabled by setting pbxt_support_xa=0, it is enabled by
>>> default.
>>>
>> Regards / Mit vielen Grüßen,
>> Sergei
>>
>> -- 
>>   __  ___     ___ ____  __
>>  /  |/  /_ __/ __/ __ \/ /   Sergei Golubchik <serg@xxxxxxx>
>> / /|_/ / // /\ \/ /_/ / /__  Principal Software Engineer/Server Architect
>> /_/  /_/\_, /___/\___\_\___/  Sun Microsystems GmbH, HRB München 161028
>>       <___/                  Sonnenallee 1, 85551 Kirchheim-Heimstetten
>> Geschäftsführer: Thomas Schroeder, Wolfgang Engels, Wolf Frenkel
>> Vorsitzender des Aufsichtsrates: Martin Häring
>
>
>
> --
> Paul McCullagh
> PrimeBase Technologies
> www.primebase.org
> www.blobstreaming.org
> pbxt.blogspot.com
>
>
>
>
Regards / Mit vielen Grüßen,
Sergei

-- 
   __  ___     ___ ____  __
  /  |/  /_ __/ __/ __ \/ /   Sergei Golubchik <serg@xxxxxxx>
 / /|_/ / // /\ \/ /_/ / /__  Principal Software Engineer/Server Architect
/_/  /_/\_, /___/\___\_\___/  Sun Microsystems GmbH, HRB München 161028
       <___/                  Sonnenallee 1, 85551 Kirchheim-Heimstetten
Geschäftsführer: Thomas Schroeder, Wolfgang Engels, Wolf Frenkel
Vorsitzender des Aufsichtsrates: Martin Häring