WL#3137: Maria - optional log partitioning

The original idea is

If we say that the log is the hot spot (the mutex to write to its end
is used 100%, assume this is a multi-CPU machine, Gray says this can
happen): we could have two logs (one on one disk, the other on
another different disk). Here it's not mirrored logs (no redundancy),
it's "log partitioning" or "log load balancing" for performance:
1) if transaction_id is even, it will write its log entries to log
file "log_even"
2) if transaction_id is odd, it will write its log entries to log
file "log_odd"
(the transaction_id odd/even thing is stable through restarts)
3) there has to be a shared counter "global LSN" so that there is an
order between entries of log_even and of log_odd (one can know which
event is before the other, needed at recovery). Atomic_add could do
it cheap.

The original idea and the discussion which followed are in the mailarchive,
Message-ID: <20060203103336.GC5813@mysql.com>