OCP-1Z0-053-V12.02-52题

52.You have the following requirements in relation to the detection of block corruption for your database
instance:
Check for logical self-consistency of data blocks when modified in memory.
Checksums are calculated before and after the block change.
Checks are performed for the lost writes to the physical standby database.
Which method would help you perform the above checks automatically?
A. Set the DB_SECUREFILE parameter to PERMITTED.
B. Set the DB_ULTRA_SAFE parameter to DATA_ONLY.

C. Set the DB_LOCK_CHECKSUM parameter to TYPICAL.
D. Set the DB_LOST_WRITE_PROTECT parameter to TYPICAL.
Answer: B

答案解析：

参考：http://docs.oracle.com/cd/E11882_01/server.112/e40402/initparams069.htm#REFRN10295

DB_ULTRA_SAFE

sets the default values for other parameters that control protection levels.
Values:

OFF

When any of

DB_BLOCK_CHECKING

,

DB_BLOCK_CHECKSUM

, or

DB_LOST_WRITE_PROTECT

are explicitly set, no changes are made.

DATA_ONLY

DB_BLOCK_CHECKING

will be set to

MEDIUM

.
http://docs.oracle.com/cd/E11882_01/server.112/e40402/initparams048.htm#REFRN10029

DB_LOST_WRITE_PROTECT

will be set to

TYPICAL

.
http://docs.oracle.com/cd/E11882_01/server.112/e40402/initparams063.htm#REFRN10268

DB_BLOCK_CHECKSUM

will be set to

FULL

.
http://docs.oracle.com/cd/E11882_01/server.112/e40402/initparams049.htm#REFRN10030

DATA_AND_INDEX

DB_BLOCK_CHECKING

will be set to

FULL

.

DB_LOST_WRITE_PROTECT

will be set to

TYPICAL

.

DB_BLOCK_CHECKSUM

will be set to

FULL

.

DB_BLOCK_CHECKING

specifies whether or not Oracle performs block checking for database blocks.

Values:

OFF

or

FALSE

No block checking is performed for blocks in user tablespaces. However, semantic block checking for

SYSTEM

tablespace blocks is always turned on.

LOW

Basic block header checks are performed after block contents change in memory (for example, after

UPDATE

or

INSERT

statements, on-disk reads, or inter-instance block transfers in Oracle RAC).

MEDIUM

All

LOW

checks and full semantic checks are performed for all objects except indexes (whose contents can be reconstructed by a drop+rebuild on encountering a corruption).

FULL

or

TRUE

All

LOW

and

MEDIUM

checks and full semantic checks are performed for all objects.

Oracle checks a block by going through the data in the block, making sure it is logically self-consistent. Block checking can often prevent memory and data corruption. Block checking typically causes 1% to 10% overhead, depending
on workload and the parameter value. Specific DML overhead may be higher. The more updates or inserts in a workload, the more expensive it is to turn on block checking. You should set

DB_BLOCK_CHECKING

to

FULL

if the performance overhead
is acceptable.
For backward compatibility, the use of

FALSE

(implying

OFF

) and

TRUE

(implying

FULL

) is preserved.

DB_LOST_WRITE_PROTECT

enables or disables lost write detection. A data block lost write occurs when an I/O subsystem acknowledges the completion of the block write,
while in fact the write did not occur in the persistent storage.

When the parameter is set to

TYPICAL

on the primary database, the instance logs buffer cache reads for read-write tablespaces in the redo log, which is necessary for detection of lost writes.

When the parameter is set to

FULL

on the primary database, the instance logs reads for read-only tablespaces as well as read-write tablespaces.
When the parameter is set to

TYPICAL

or

FULL

on the standby database or on the primary database during media recovery, the instance performs lost write detection.
When the parameter is set to

NONE

on either the primary database or the standby database, no lost write detection functionality is enabled.

DB_BLOCK_CHECKSUM

determines whether

DBW

n

and
the direct loader will calculate a checksum (a number calculated from all the bytes stored in the block) and store it in the cache header of every data block when writing it to disk. Checksums are verified when a block is read - only if this parameter is

TYPICAL

or

FULL

and
the last write of the block stored a checksum. In

FULL

mode, Oracle also verifies the checksum before a change application from update/delete statements and recomputes it after the change is applied. In
addition, Oracle gives every log block a checksum before writing it to the current log.

Starting with Oracle Database 11g, most of the log block checksum is done by the generating foreground processes, while the LGWR performs the rest of the work, for better CPU and cache efficiency. Prior to Oracle Database 11g,
the LGWR solely performed the log block checksum.
If this parameter is set to

OFF

,

DBW

n

calculates checksums only for the

SYSTEM

tablespace, but not for user tablespaces. In addition, no log checksum is performed when this parameter
is set to

OFF

.
Checksums allow Oracle to detect corruption caused by underlying disks, storage systems, or I/O systems. If set to

FULL

,

DB_BLOCK_CHECKSUM

also catches in-memory corruptions and stops them from making
it to the disk. Turning on this feature in

TYPICAL

mode causes only an additional 1% to 2% overhead. In the

FULL

mode it causes 4% to 5% overhead. Oracle recommends that you set

DB_BLOCK_CHECKSUM

to

TYPICAL

.
For backward compatibility the use of

TRUE

(implying

TYPICAL

) and

FALSE

(implying

OFF

) values is preserved.