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Logical Volume Manager

The database is constructed and maintained on file system files or raw partitions, which are entities at the operating system level. They are directly attached to storage devices, NAS devices, or SAN volumes.

There are several different layers of abstraction between the physical disks and the objects that the database uses. The physical drive is at the lowest level, next come the LUNS (logical unit numbers) of manageable sizes employing an appropriate RAID level. LUNS are presented to the host system as physical objects.

The Logical Volume Manager (LVM), a software module running at the host level, manages the physical objects and presents them as logical units that the application can use. The volume manager hides the physical attributes of the disks or LUNs by introducing virtualization. The volume manager can also execute the RAID operations if needed. Within the volume manager, storage devices are grouped into disk groups (e.g. Veritas disk group).

Within a disk group, we can create a logical volume with an optional or desired RAID level. Finally, the logical volume is either presented as a file system after mounting, or presented as a raw partition for application use. The VM tool provides a very flexible way of managing the volumes and file systems. Without a volume manager, LUNS are physical disks with partitions.

Fig 5.7 Volumes and File system with Volume Manager

Thus the volume manager hides the details about where data is stored in the hardware from the entire system. Volume management lets us edit the storage configuration, for example, setting up a software RAID or extending volume size, etc., without actually changing anything on the hardware side. Some volume manager tools allow snapshot copies to be taken of volumes to move or backup. Thus, for either creating the raw partitions (or volumes) or setting up of file systems, the volume manager comes as a handy tool.

The Veritas volume manager has been in use on many platforms like Solaris, HP, and AIX. Now, Veritas even has the Volume Manager to support Windows systems. On the Linux side, SuSE LVM has been in use for a long time.

Fig 5.7 above shows how the LVM provides volumes and file systems to a host. LVM hides all the complexity in the backend of the disk drives or the storage units. The database system will just interact with the volumes presented by the LVM.


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