Logical-volume manager

You can use the logical-volume manager (LVM) utility to manage your disk space through user-defined logical volumes.

Many computer manufacturers ship their computers with a proprietary LVM. You can use the database server to store and retrieve data on disks that are managed by most proprietary LVMs. Logical-volume managers provide some advantages and some disadvantages, as explained in the remainder of this section.

Most LVMs can manage multiple gigabytes of disk space. The database server chunks are limited to a size of 4 terabytes, and this size can be attained only when the chunk being allocated has an offset of zero. Consequently, you must limit the size of any volumes to be allocated as chunks to a size of 4 terabytes.

Because you can use LVMs to partition a disk drive into multiple volumes, you can control where data is placed on a given disk. You can improve performance by defining a volume that consists of the middle-most cylinders of a disk drive and placing high-use tables in that volume. (Technically, you do not place a table directly in a volume. You must first allocate a chunk as a volume, then assign the chunk to a dbspace, and finally place the table in the dbspace. For more information, see Control of where simple large object data is stored.)
Tip: If you choose to use large disk drives, you can assign a chunk to one drive and eliminate the necessity to partition the disk.

You can also improve performance by using a logical volume manager to define a volume that spreads across multiple disks and then placing a table in that volume.

Many logical volume managers also allow a degree of flexibility that standard operating-system format utilities do not. One such feature is the ability to reposition logical volumes after you define them. Thus getting the layout of your disk space right the first time is not so critical as with operating-system format utilities.

LVMs often provide operating-system-level mirroring facilities. For more information, see Alternatives to mirroring.


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