System Management Guide:
Operating System and Devices

Define a Raw Logical Volume for an Application

A raw logical volume is an area of physical and logical disk space that is under the direct control of an application, such as a database or a partition, rather than under the direct control of the operating system or a file system. Bypassing the file system can yield better performance from the controlling application, especially from database applications. The amount of improvement, however, depends on factors such as the size of a database or the application's driver.

Note

You will need to provide the application with the character or block special device file for the new raw logical volume, as appropriate. The application will link to this device file when it attempts opens, reads, writes, and so on.

Attention: Each logical volume has a logical-volume control block (LVCB) located in the first 512 bytes. Data begins in the second 512-byte block. In a raw logical volume, the LVCB is not protected. If an application overwrites the LVCB, commands that normally update the LVCB will fail and generate a message. Although the logical volume might continue to operate correctly and the overwrite can be an allowable event, overwriting the LVCB is not recommended.

The following instructions use SMIT and the command line interface to define a raw logical volume. You can also use the Create a new logical volume wizard in Web-based System Manager (select Volumes -> Overview and Tasks -> Create a new logical volume). To define a raw logical volume within the wizard, accept the default use, applications and data, from its first selection screen. Online help is available if you need it.

With root authority, find the free physical partitions on which you can create the raw logical volume by typing the following SMIT fast path:
```
smit lspv
```
Select a disk.
Accept the default in the second dialog (status) and click OK.
Multiply the value in the FREE PPs field by the value in the PP SIZE field to get the total number of megabytes available for a raw logical volume on the selected disk. If the amount of free space is not adequate, select a different disk until you find one that has enough available free space.
Exit SMIT.
Use the mklv command to create the raw logical volume. The following command creates a raw logical volume named lvdb2003 in the db2vg volume group using 38 4-MB physical partitions:
```
mklv -y lvdb2003 db2vg 38
```
Use the -y flag to provide a name for the logical volume instead of using a system-generated name.

At this point, the raw logical volume is created. If you list the contents of your volume group, a raw logical volume is shown with the default type, which is jfs. This type entry for a logical volume is simply a label. It does not indicate a file system is mounted for your raw logical volume.

Consult your application's instructions on how to open /dev/rawLVname and how to use this raw space.

System Management Guide: Operating System and Devices

Define a Raw Logical Volume for an Application

System Management Guide:
Operating System and Devices