Introduction To LVM

Logical Volume manager (LVM):

Dictionary Definition: LVM is a set of commands, library routines and tools used to allow and control logical volume storage.

LVM helps in spreading data on logical volumes across multiple physical volumes. Noncontiguous data on physical volume appears to be contiguous on logical volumes.

What does LVM gives:

–          Flexibility (expansion / deletion is possible. User can customize the logical volumes)

–          Speed

–          Data integrity

–          Resolves noncontiguous disk problems

–          Separate user data

–          Size of file systems can be increased / decreased as per user requirements.

–          LV can span across multiple PV

–          LV size can be dynamically increased

–          Mirroring of LV is possible

Limitations of LVM:

–          Reduction of size of LV is not possible.

–          Moving of data across VG is not possible.

Terms associated with LVM are:

Physical Volume (PV): The physical hard disk available in AIX Box. PV is located inside the VG.

Logical Volume (LV): Data is stored on LV which is in turn stored on PV.

Physical Partition (PP): data on PV is stored in small units called PP.

Logical partition (LP): Data on LV is stored in smaller units called LP.

Volume Group Descriptor Area (VGDA): it contains information on what hard disk make up the Volume group.

Volume group (VG): LV stores data on Volume group.

Volume Group Status Area (VGSA): contains information about the stale and missing PP.

Logical Volume Control Block (LVCB): it contains information on creation of LV, LP allocation and information about mirrored copies and mount points on JFS.

Quorum: validation technique done by LVM to resolve data corruption. Quorum checking is done on the basis of VGDA information available in VG. Greater than 51% votes are needed to allow valid quorum checking. A quorum is lost when VGDA information is unavailable / unreachable and does not contribute to 51% of VGDA quota.

Points to Remember:

Size of LP is equal to that of PP. higher the number of PP’s allocated, lower is the performance.

LP and PP are always configured in 1:1, 1:2, 1:3 ratio (respectively) depending on the disk mirroring done.

LP    :    PP

1      :     1

1      :     2

1      :     3

Mirroring of data increases the availability of data but reduces the performance. A mirrored LV is slower in performance as compared to unmirrored LV as the data is written in 2 or 3 places.

To avoid disruptions from failures like power failure or adapter failure, ideal configuration to be used is: 2 adapters, non quorum VG configuration, at least 1 disk per adapter & mirroring across adapters.