RE: RMAN: What blocks are backed up with a full backup?

But it does make sense that when ASSM is in use that RMAN would read the bitmap,
doesn't it. Something for me to ask the Oracle RMAN folks.

RF

Robert G. Freeman
Technical Management Consultant
TUSC - The Oracle Experts www.tusc.com
904.708.5076 Cell (It's everywhere that I am!) Author of several books you can find on Amazon.com!

-----Original Message-----
Sent: Sunday, April 06, 2003 10:44 AM
To: Multiple recipients of list ORACLE-L

I partially agree with Mladen.

RMAN definitely doesn't identify Table HWMs -- it doesn't know logical structures such as Tables and Segments and Tablespaces [although it can assist you in Tablespace Recovery when it reads the Data Dictionary]. RMAN knows physical structures such as DataFiles, RedoLog Files and ControlFiles. When backing up a DataFile, RMAN would ignore __unused__ blocks --- blocks that have never been used. Now, the part where I don't agree with Mladen is about reading the bitmap information from the tablespace header. RMAN can backup DataFiles belonging to Dictionary Managed Tablespaces without Automatic Segment Space Management. In such cases there is no bitmap in the tablespace header to read from.

When Oracle creates a DataFile it initializes the file with NULLs. When a block is used [written to as a Table Row-Block or an Index Leaf-Block], the block has to be formatted before it is written-to. Once it is formatted [the formatting is the block header, table column names, and possibly the initrans area and pctfree area], the block will always remain "used" even if all the rows in it are deleted.

Thus, when RMAN goes through a DataFile it examines each block and ignores unused blocks which have not been formatted.

Such unused or empty blocks may be within an Extent or unallocated to any extent !

Hemant

At 08:13 AM 04-04-03 -0800, you wrote:
>What it doesn't write are the blocks that are not allocated to any
>extent. RMAN doesn't go into the logical structures, like tables and
>indexes,
>it looks into the tablespace header and reads the information from the
>bitmap information there. It cannot go into tables/indexes because it
>should also work when the database is only mounted and not opened, which
>genrally means that data dictionary is not accessible. Empty blocks
>are blocks that don't have any rows in them but are allocated to an
>extent. New or unallocated blocks are blocks that have been initialized
>when the datafile was allocated to the tablespace but have not been
assigned
>to any object (table, index, materialized view, cluster, partition or
alike)
>
>-----Original Message-----
>Sent: Thursday, April 03, 2003 4:24 PM
>To: Multiple recipients of list ORACLE-L
>
>
>Actually this is how RMAN works. It writes all blocks up to the HWM of a
>given table,
>even empty ones. So, if your HWM is artifically high, you will encounter
>backups
>that are larger than they need to be.
>
>Oracle9i RMAN Backup and Recovery.... On bookshelves now!
>
>RF
>
>
>-----Original Message-----
>To: Multiple recipients of list ORACLE-L
>Sent: 4/3/2003 11:19 AM
>
>Keith
> Here is my understanding (don't rely on this one). When Oracle
>allocates
>tablespace, the disk blocks are cleared. My interpretation is that when
>RMAN
>encounters a clear block, it doesn't write it to the backup piece. I
>don't
>think it spends a lot of time trying to figure out above HWM and such.
>
>Dennis Williams
>DBA, 40%OCP, 100% DBA
>Lifetouch, Inc.
>dwilliams_at_lifetouch.com
>
>
>-----Original Message-----
>Sent: Thursday, April 03, 2003 10:44 AM
>To: Multiple recipients of list ORACLE-L
>
>
>Our RMAN backup is backing up much more than the actual data blocks when
>doing a full backup. I know that it backs up all blocks that have "ever
>been
>used", but I'm trying to figure out exactly what that means. My first
>thought was that it backs up all blocks below the HWM, but I analyzed
>the
>tables and that is not the case.
>
>Sometimes it backs up more blocks than exist below the HWM for the
>tables
>and sometimes it backs up fewer blocks than those below the HWM.
>
>We are doing this to determine what we can do to reduce the size of the
>backup.
>
>Anyone have an idea how this works?
>
>Keith
>
>
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Hemant K Chitale
My personal web site is : http://hkchital.tripod.com

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