What happened when tablespace/database placed into backup mode?

User managed backup or physical backup without RMAN is still one of the popular method for backup and still part of mostly disk to disk backup methods (BCV or SRDF copy). Today I am going to discuss what happens when we place tablespace/database into backup mode.

Fractured Block

Unit of I/O for any system is block size and for oracle database it’s 2K/4K/8K/16K/32K while for any operating system it could be lower than that. One risk in making online backups is the possibility of inconsistent data within a block. Assume that OS I/O block size is 512 bytes while we used DB block size as 8K.

This means to copy utility of OS would take 16 I/Os to take backup of one DB block, while during the backup, DBWR could write 8K in one I/O.

So assume a scenario where copy utility is busy taking backup of 8th portion of particular block while DBWR came to overwrite same DB block (i.e. all 16 OS block pieces in one DB block). Because copy utility was still busy on taking backup of remaining blocks so it took new image from OS piece 9th onwards. From database perspective that backup copy of db block (collection of 16 OS blocks) has been fractured as its half or the OS blocks are old and remaining half are new. The result is called a fractured block, meaning that the data contained in this block is not consistent at a given SCN.

Oracle’s internal protection mechanism against fractured block

To overcome this danger Oracle introduced a protection feature which will be activate once you trigger ‘alter tablespace/database begin backup’ command. Following internal operations happened with immediate effect after triggering begin backup for tablespace Read more of this post

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Redo Block Size

Redo logs or online redo log files are crucial files for any oracle database to be operational with durability of transaction. All change vectors must be written to current redo log (except nologging and few exceptional settings where database is running purely for performance benchmark purpose)

Redo log files always default to a block size that is equal to the physical sector size of the disk. Historically, this has typically been 512 bytes (512B). Beginning with Oracle Database 11g Release 2, you can specify the block size of online redo log files with the BLOCKSIZE keyword in the CREATE DATABASE, ALTER DATABASE, and CREATE CONTROLFILE statements. The permissible block sizes are 512, 1024, and 4096.

So we are going to check if we can try with this cool option. Let’s check the current block size of existing redo log files from block_size column of v$log view.

Read more of this post

Can I Clear Current Redo Logfile while it’s required for recovery??

Normally we need to clear redo log if its corrupted and oracle is not able to reuse it. It may already be archived or may not be. In both cases we need to clear it to allow oracle instance to re-use it. You have to make active redolog to inactive by manual checkpoint or by switching logfile (if possible).

Commands would be

ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP <group number>;

ALTER DATABASE CLEAR LOGFILE GROUP <group number>;

In case of corruption in current redo log following two scenarios could take place

1) Database instance crashed with Ora- 600 error; You can’t do much as now you need to check your last successful backup and all archives/incremental backup to perform incomplete recovery.

or

2)  Database still up and running (chances are less but possible); So could we try to clear the corrupted current redo log??

Let’s try to mimic the situation (except corruption)

Read more of this post

Little insight of child cursor part -1

Simple queries against demo schema HR under 10g database.

==============================================

SQL> select * from emp,dept where emp.ename=’SMITH’ and dept.deptno=emp.deptno;

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO DEPTNO DNAME LOC
———- ———- ——— ———- ——— ———- ———- ———- ———- ————– ————-
7369 SMITH CLERK 7902 17-DEC-80 800 20 20 RESEARCH DALLAS

SQL> select * from emp,dept where emp.ename=’CLARK’ and dept.deptno=emp.deptno;

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO DEPTNO DNAME LOC
———- ———- ——— ———- ——— ———- ———- ———- ———- ————– ————-
7782 CLARK MANAGER 7839 09-JUN-81 2450 10 10 ACCOUNTING NEW YORK

SQL> select * from emp,dept where emp.ename=’AMIT’ and dept.deptno=emp.deptno;

no rows selected

———————————————————————————————

Listing of child cursor attached to same parent cursor. Oracle creates child cursor for every set of bind variable pair. In that case I was changing ename with SMITH, CLARK, & AMIT. So every time oracle created one child cursor to hold bind variables. Oracle tries to prepare a different execution plan for

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