Re: why r more archives generated during hot backup

Hi Cane001,

To protect the database from "fractured blocks".

As you're performing an OS copy at the same time Oracle is writing to the data files, the possibility exists that a portion of an Oracle block is copied, followed by the DBWR overwriting the particular block , followed by the remaining portion of the Oracle block being copied. This means the copied Oracle block has a split personality in that one portion of the block is from one point in time and another portion of the block is from a different point of time.

In other words, a fractured block.

Restoring such a block would prove to be unrecoverable with the usual generation of redo as there is no way to reconstruct the block in a consistent manner.

Therefore, while a data file is in hot backup mode, Oracle prevents issues with fractured blocks by coping the before/after image of the entire block such that an entire consistent block can be used during the recovery process.

This is one of the reasons why it's vital to place the tablespace in backup mode before attempting a hot backup.

This excessive redo generation is also the reason why it's recommended where possible to place one tablespace at a time in backup mode and why hot backups should be performed during a quiet period.

Note that RMAN doesn't concern itself with this issue as fractured blocks are not possible with RMAN. That's because RMAN performs a checksum test on each block as it performs the copy. If it fails this test, RMAN simply recopies the block. If it passes the test this time, it was obviously a fractured block which the immediate recopy has automatically remedied. If it fails for a second time, then it's a corrupted block which RMAN duly documents.

Cheers

Richard
"Cane001" <nkapadia41_at_yahoo.com> wrote in message news:acf7ab77.0209200004.72143f7f_at_posting.google.com...
> I would like to know why r more archives generated during hot backups
Received on Fri Sep 20 2002 - 03:37:25 CDT