It perform the below steps in details…..
1. Oracle checkpoints the tablespace, also flushing all
changes from shared memory to disk.
2. The SCN markes for each datafile in that tablespace as “frozen” at their current values.
But further updates
will be sent to the datafiles, the SCN markers will not be updated until the tablespace is
taken out of backup mode.
3.Then Oracle
switches to logging full images of changed database blocks to the redologs.
Instead of recording how it changed a particular block the change vector), it
will log the entire image of the block after he change. This is why the redologs
grow at a much faster rate while hot backups are going on.
What is ‘Split –
Block ‘ ?
After this, your
backup program works happily through this datafile, backing it up block by
block. Since the file is being updated as you are reading it, it may read
blocks just before they’re changed, after they’re changed, or even while
they’re changing.
Suppose that your
filesystem block size is 4 KB, and Oracle’s block size is 8 KB. Your backup
program will be reading in increments of 4 KB. It could back up the first 4 KB
of an 8-KB Oracle data block before a change is made to that block, then back
up the last 4 KB of that file after a change has been made. This results in
what Oracle calls a “split block”.
However, when your backup program reaches the point of the
datafile that contains the SCN, it will back up that block the way it looked
when the backup began, since that block is frozen. Once you take the tablespace
out of backup mode, the SCN marker is advanced to the current value, and Oracle
switches back to logging change vectors instead of full images of changed
blocks.
See Also : OS Interview for DBA
No comments:
Post a Comment