Database Reference
In-Depth Information
For my environment here is the code generated that can be executed from SQL*Plus to copy the data files back
from the backup directory, if a failure should occur:
!cp /u01/hbackup/O12C/system01.dbf /u01/dbfile/O12C/system01.dbf
!cp /u01/hbackup/O12C/sysaux01.dbf /u01/dbfile/O12C/sysaux01.dbf
!cp /u01/hbackup/O12C/undotbs01.dbf /u01/dbfile/O12C/undotbs01.dbf
!cp /u01/hbackup/O12C/users01.dbf /u01/dbfile/O12C/users01.dbf
!cp /u01/hbackup/O12C/tools01.dbf /u01/dbfile/O12C/tools01.dbf
In this output, you can remove the exclamation point (
!
) from each line if you prefer to run the commands from
the OS prompt. The main idea is that these commands are available in the event of a failure, so you know which files
have been backed up to which location and how to copy them back.
■
don't use user-managed hot backup technology for online backups; use rMan. rMan doesn't need to place
tablespaces in backup mode and automates nearly everything related to backup and recovery.
Tip
Understanding the Split-Block Issue
To perform a hot backup, one critical step is to alter a tablespace into backup mode before you copy any of the data
files associated with the tablespace, using an OS utility. To understand why you have to alter a tablespace into backup
mode, you must be familiar with what is sometimes called the split- (or fractured-) block issue.
Recall that the size of a database block is often different from that of an OS block. For instance, a database block
may be sized at 8KB, whereas the OS block size is 4KB. As part of the hot backup, you use an OS utility to copy the live
data files. While the OS utility is copying the data files, the possibility exists that database writers are writing to a block
simultaneously. Because the Oracle block and the OS block are different sizes, the following may happen:
1.
The OS utility copies part of the Oracle block.
2.
A moment later, a database writer updates the entire block.
3.
A split second later, the OS utility copies the latter half of the Oracle block.
This can result in the OS copy of the block's being inconsistent with what Oracle wrote to the OS. Figure
3-1
illustrates this concept.
