SQL> show parameter fast_start

NAME                                 TYPE        VALUE
------------------------------------ ----------- ----------------------
fast_start_mttr_target               integer     71

-- Here you can see that value of parameter "fast_start_mttr_target" = 71.
-- This is best MTTR which the system can achieve for my system 

SQL> select * from v$log;

    GROUP#    THREAD#  SEQUENCE#      BYTES    MEMBERS ARC STATUS 
---------- ---------- ---------- ---------- ---------- --- ------- 
         1          1        485   10485760          1 NO  INACTIVE
         2          1        486   10485760          1 NO  INACTIVE
         3          1        487   10485760          1 NO  CURRENT

-- Initially the size of redo logs are 10 MB

SQL> select ACTUAL_REDO_BLKS,TARGET_REDO_BLKS,TARGET_MTTR,ESTIMATED_MTTR,
    OPTIMAL_LOGFILE_SIZE,CKPT_BLOCK_WRITES  from v$instance_recovery;

ACTUAL_REDO_BLKS TARGET_REDO_BLKS TARGET_MTTR ESTIMATED_MTTR OPTIMAL_LOGFILE_SIZE
---------------- ---------------- ----------- -------------- --------------------
          942            18432          71             59                     49      

-- The recommended optimal redolog file size is 49 MB as seen from column -OPTIMAL_LOGFILE_SIZE.
-- This is as per the setting of "fast_start_mttr_target" = 71.

SQL>  select ACTUAL_REDO_BLKS       ,TARGET_REDO_BLKS,TARGET_MTTR,ESTIMATED_MTTR,
  2   OPTIMAL_LOGFILE_SIZE,CKPT_BLOCK_WRITES       from v$instance_recovery;

ACTUAL_REDO_BLKS TARGET_REDO_BLKS TARGET_MTTR ESTIMATED_MTTR  OPTIMAL_LOGFILE_SIZE
--------------- ---------------- ----------- --------------  --------------------
            597            18432          71             59                   49         


-- Here redo logs are re-created as per recommendations 49 MB

SQL> select * from v$log;

    GROUP#    THREAD#  SEQUENCE#      BYTES    MEMBERS ARC STATUS
---------- ---------- ---------- ---------- ---------- --- ----------------
         4          1        490   51380224          1 NO  INACTIVE
         5          1        491   51380224          1 NO  ACTIVE
         6          1        492   51380224          1 NO  CURRENT


SQL> select ACTUAL_REDO_BLKS       ,TARGET_REDO_BLKS,TARGET_MTTR,ESTIMATED_MTTR,
  2  OPTIMAL_LOGFILE_SIZE,CKPT_BLOCK_WRITES       from v$instance_recovery;

ACTUAL_REDO_BLKS TARGET_REDO_BLKS TARGET_MTTR ESTIMATED_MTTR  OPTIMAL_LOGFILE_SIZE
---------------- ---------------- ----------- --------------  --------------------
             113            18432          71             58                    49       


-- You can see that the "actual_redo_blks" column, that is current number of redo blocks 
-- required to be read for recovery has reduced from 597 redo blocks earlier to 113
-- redo blocks once the log files are re-created with optimal settings.
--
-- This will speed up the Instance recovery time.
--