does-startup-mount-verify-datafiles

There seems to be a misunderstanding that a MOUNT actually verifies datafiles.
A STARTUP MOUNT  (or STARTUP NOMOUNT followed by ALTER DATABASE MOUNT) does *NOT* read the datafiles and/or verify them.  It does read the controlfile(s).

Here's a simple test :

I have a tablespace with a datafile that is "ONLINE".

SQL> create tablespace X_TBS datafile '/tmp/X_TBS.dbf' size 50M;

Tablespace created.

SQL> create table hemant.X_TBS (col_1) tablespace X_TBS
  2  as select rownum from dual connect by level < 100;

Table created.

SQL> 

SQL> select file#, status, name 
  2  from v$datafile
  3  where name like '%X_TBS%';

     FILE# STATUS  NAME
---------- ------- ----------------------------------------
        14 ONLINE  /tmp/X_TBS.dbf

SQL> select file#, status, name
  2  from v$datafile_header
  3  where name like '%X_TBS%';

     FILE# STATUS  NAME
---------- ------- ----------------------------------------
        14 ONLINE  /tmp/X_TBS.dbf

SQL> select owner, segment_name, bytes/1024
  2  from dba_segments
  3  where tablespace_name = 'X_TBS';

OWNER
------------------------------
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES/1024
----------
HEMANT
X_TBS
        64


SQL> 

I now shutdown the database instance and remove the datafile :

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> !ls /tmp/X_TBS.dbf
/tmp/X_TBS.dbf

SQL> !rm /tmp/X_TBS.dbf

SQL> !ls /tmp/X_TBS.dbf
ls: /tmp/X_TBS.dbf: No such file or directory

SQL> 

Does the STARTUP MOUNT succeed ?

SQL> startup mount
ORACLE instance started.

Total System Global Area  535662592 bytes
Fixed Size                  1337720 bytes
Variable Size             213911176 bytes
Database Buffers          314572800 bytes
Redo Buffers                5840896 bytes
Database mounted.
SQL>
SQL> shutdown
ORA-01109: database not open


Database dismounted.
ORACLE instance shut down.
SQL> startup nomount;
ORACLE instance started.

Total System Global Area  535662592 bytes
Fixed Size                  1337720 bytes
Variable Size             213911176 bytes
Database Buffers          314572800 bytes
Redo Buffers                5840896 bytes
SQL> alter database mount;

Database altered.

SQL> 

Can the file be listed ? Yes. However, V$DATAFILE_HEADER no longer shows the name ! The query on V$DATAFILE_HEADER does cause Oracle to "look" for the file but it does NOT cause a failure. It simply finds it "missing".

SQL> select file#, status, name
  2  from v$datafile
  3  where file#=14;

     FILE# STATUS  NAME
---------- ------- ----------------------------------------
        14 ONLINE  /tmp/X_TBS.dbf

SQL> select file#, status, name
  2  from v$datafile_header
  3  where file#=14;

     FILE# STATUS  NAME
---------- ------- ----------------------------------------
        14 ONLINE

SQL> 

When does Oracle attempt to access the datafile ?

SQL> alter database open;
alter database open
*
ERROR at line 1:
ORA-01157: cannot identify/lock data file 14 - see DBWR trace file
ORA-01110: data file 14: '/tmp/X_TBS.dbf'


SQL> 

Even as the OPEN failed with an ORA-01157, the datafile is present in the controlfile :

SQL> select file#, status, name
  2  from v$datafile
  3  where file#=14;

     FILE# STATUS  NAME
---------- ------- ----------------------------------------
        14 ONLINE  /tmp/X_TBS.dbf

SQL> select file#, status, name
  2  from v$datafile_header
  3  where file#=14;

     FILE# STATUS  NAME
---------- ------- ----------------------------------------
        14 ONLINE

SQL> 

I hope that I have you convinced that a MOUNT does NOT verify the datafiles. If you are still not convinced, read the Backup and Recovery documentation about how to do a FULL DATABASE RESTORE and RECOVER -- where you restore the controlfile and mount the database before you even restore datafiles. How would the MOUNT succeed with the controlfile alone ?