Logical volume control block

Background

A raw logical volume is a physical partition that is not directly controlled by
AIX and the file system. Usually it is for use with databases that need better
performance than they would normally get with file systems.

Whatever the reason for using a raw logical volume, you must remember that AIX
has the ability to allow a database program to use a raw logical volume for
storing data, but expects that database program or utilities for that program
to manage the data stored in that location. AIX data management tools are
designed for working at the file system level, which is one level above the
logical volume level.

WARNING: Please note that due to the nature and differences of the databases
available today, IBM does not guarantee that any data in a database (raw or
otherwise) will be recoverable using any AIX commands.

--------------------------------------------------------------------------------

Logical volume control block

Every AIX logical volume has a 512-byte block at the beginning of the LV called
the Logical Volume Control Block (LVCB). The LVCB keeps track of information in
the logical volume. Some database vendors have chosen to write over the LVCB and
use their own methods of keeping track of the information in the LV.

When using the AIX dd command for archiving and retrieving raw logical volumes,
it is important to know if your database vendor uses the AIX LVCB or writes over
it. This information is important and is referenced later in this document.

--------------------------------------------------------------------------------

dd limitations

The AIX dd command has some limitations which must be taken into consideration
prior to archiving a raw logical volume.

The dd command cannot span multiple tapes. You must be able to fit the entire
raw logical volume onto one tape. If you require multiple tapes, your only AIX
option for archiving a raw logical volume is to use SYSBACK 6000.

The dd command in AIX 3.2 and 4.1 cannot archive a single file (or logical
volume) greater than 2GB. In AIX 4.2, the dd command has been designed to archive
data greater than 2GB in size.

--------------------------------------------------------------------------------

Steps to archive raw logical volumes
Decide on the appropriate tape device blocksize.

To check the device blocksize, execute the following command:
tctl -f /dev/rmt0 status

To change a device blocksize, execute the following command:
chdev -l rmt0 -a block_size=

Recommended values are:

9track/ 1/4in = 512
8mm/4mm/dlt = 1024


Create an archived raw logical volume.

NOTE: When you use the conv=sync flag, all reads that are smaller than the ibs
value will be padded to equal the ibs value. This can greatly affect files
sensitive to change, such as database files.

For example:

ibs=512; file filesize = 52 bytes
52 bytes + 460 blanks = 512 bytes


To archive without software compression, run the following command:
dd if= of=/dev/rmt0 ibs=512 obs= conv=sync

To archive with software compression, run the following command:
dd if= bs=512 | compress | \
dd of=/dev/rmt0 ibs=512 obs= conv=sync

Restoring a raw logical volume archive.

To restore a raw logical volume archive we must know whether or not to overwrite
the Logical Volume Control Block. For more info on the LVCB, see the section
"Logical volume control block".

NOTE: The skip=1 allows the read function to skip over one 512-byte block on the
tape. The seek=1 allows the write function to skip over one 512-byte block on
the disk.

Using the Current System LVCB

To restore without software compression, run the following command:
dd if=/dev/rmt0 ibs= obs=512 | \
dd of=/dev/ bs=512 skip=1 seek=1

To restore with software compression, run the following command:
dd if=/dev/rmt0 ibs= obs=512 | \
uncompress | dd of=/dev/ bs=512 skip=1 seek=1

Overwriting the Current System LVCB
WARNING: You must NOT overwrite the LVCB unless you are certain you need to.

To restore without software compression, run the following command;
dd if=/dev/rmt0 of=/dev/ ibs obs=512

To restore with software compression, run the following command:
dd if=/dev/rmt0 ibs= obs=512 | \
uncompress | dd of=/dev/ bs=512