VxVM Volumes

VxVM uses logical volumes to organize and manage disk space. A volume is made up of portions of one or more physical disks, so it does not have the physical limitations of a physical disk.

A volume can provide greater capacity and better availability and performance than a single physical disk. A volume can be extended (grown) across multiple disks to increase capacity, mirrored (copied) on another disk to provide data redundancy, and/or striped across multiple disks to improve I/O performance.

You can use VxVM to create the following types of volumes:

Concatenated Volume
A concatenated volume is made up of one or more disk regions that are linked together (concatenated) in a linear fashion. A concatenated volume can consist of disk regions on multiple disks; a concatenated volume that extends across two or more disks is also known as a spanned volume.
NOTE: Data in this type of volume cannot be recovered if the underlying disk fails. However, a concatenated volume can be mirrored (copied) onto other disks to protect its data against disk failure.

Striped Volume

Striped volume data is interleaved (striped) across two or more physical disks. Striped volume data is spread across the disks alternately and evenly in small, equal-sized portions of data called stripe units. Striping improves performance.




	NOTE: Data in this type of volume cannot be recovered if one of the underlying disks fails. However, a striped volume can be mirrored (copied) onto other disks to protect its data against disk failure.

RAID-5 Volume
RAID-5 volume data is interleaved (striped) across three or more physical disks. Within each stripe across the set of disks, the data on one of the disks is parity data. If one of the physical disks fails, the parity data can be used to reconstruct and recover the lost data.
NOTE: RAID-5 volumes cannot be mirrored.

Mirrored Volume
Volumes with concatenated or striped layouts can be mirrored to increase data availability. All of the data in a mirrored volume is duplicated on at least one other physical disk. If one of the disks fails, the data can still be accessed from one of the remaining disks.
The plexes in a mirrored volume typically have the same layout, but a volume can consist of plexes with different layouts. A mirrored volume has a "mixed" layout if the plexes in the volume have different layouts.
Layered Volume
A layered volume is built on one or more other volumes. The underlying volumes are typically mirrored. In layered volumes, mirroring is done at a lower level and with smaller granularity than with non-layered volumes, so each mirror covers a relatively small storage region.
Layered volumes tolerate disk failure better than non-layered volumes and provide improved data redundancy. If a disk in a layered volume fails, only a portion of the redundancy is lost and recovery time is usually quicker than it would be for a non-layered volume. Layered volumes also reduce the chance that two disk failures will result in lost data.
The underlying volumes in a layered volume are used exclusively by VxVM and are not intended for user manipulation.
With VxVM, you can create the following types of layered volumes:
- Concatenated Pro Volume
  A concatenated pro volume is a layered volume that concatenates several underlying mirror volumes.
- Striped Pro Volume
  A striped pro volume is a layered volume that stripes several underlying mirror volumes.

The following table summarizes the advantages and disadvantages of each volume layout.

Table 4-1 Layout Comparison

Layout	Advantages	Disadvantages
Concatenated	The volume can consist of disk regions that are not adjacent. The volume can span multiple disks. A spanned volume can have a greater capacity than a single disk. The volume can be mirrored to protect data against disk failure and reduce the risk of data loss.	There is a risk of data loss if the volume is not mirrored. Spanning a volume across multiple disks increases the chance that a disk failure will result in the failure of the volume. Concatenation does not improve I/O performance.
Striped	Striping provides improved read and write performance. Striping can help to balance the I/O load from multi-user applications across multiple disks. The volume can be mirrored to protect data against disk failure.	There is a risk of data loss if the volume is not mirrored. Striping a volume across multiple disks increases the chance that a disk failure will result in the failure of the volume. A striped volume requires at least two disks.
RAID-5	RAID-5 provides protection against disk failure and reduces the risk of data loss. RAID-5 provides data redundancy by storing parity (a calculated value) on the disks. If a disk fails, the parity is used to reconstruct the missing data. RAID-5 requires less storage space than mirroring. RAID-5 provides improved read performance.	RAID-5 provides relatively slow write performance. RAID-5 volumes cannot be mirrored. A RAID-5 volume requires at least three disks. If logging is enabled, a RAID-5 volume requires at least four disks. A RAID-5 volume cannot survive multiple disk failures. However, optional RAID-5 logs reduce this risk. If one of the volume disks is inaccessible, performance is degraded.
Mirrored	Mirroring provides protection against disk failure and reduces the risk of data loss. Mirroring provides data redundancy by maintaining multiple complete copies of volume data on different disks. Mirroring provides improved read performance.	A mirrored volume requires extra disk space. Each mirror requires enough disk space to contain a complete copy of the volume.
Concatenated Pro	The volume is layered, and concatenates several underlying mirror volumes. This provides improved data redundancy and reduces the impact of disk failures. This type of volume is also known as a concatenated-mirror volume. Recovery time is quicker than with regular concatenated volumes that are mirrored. The volume can consist of disk regions that are not adjacent. The volume can span multiple disks. A spanned volume can have a greater capacity than a single disk.	A concatenated pro volume requires at least two disks. Concatenation does not improve I/O performance. Layered volumes consist of more VxVM objects than non-layered volumes and are likely to fill up the disk group configuration database sooner. When the configuration database is full, you cannot create more volumes in the disk group.
Striped Pro	The volume is layered, and stripes several underlying mirror volumes. This provides improved data redundancy and reduces the impact of disk failures. This type of volume is also known as a striped-mirror volume. Recovery time is quicker than with regular striped volumes that are mirrored. Striping provides improved read and write performance. Striping can help to balance the I/O load from multi-user applications across multiple disks.	A striped pro volume requires at least four disks. Layered volumes consist of more VxVM objects than non-layered volumes and are likely to fill up the disk group configuration database sooner. When the configuration database is full, you cannot create more volumes in the disk group.




	NOTE: The total volume size you indicate for the volume in the Create Volume Wizard is the usable size for storing the data. Depending on the layout type, the volume can take more space on the disk. The size shown in the Wizard for simple, striped, and spanned volumes is the actual volume size on the disk. A RAID-5 volume requires additional space for parity information, and a mirrored volume is a multiple of the space taken by the original plex of the volume. The Wizard provides a Query Max Size button to determine the size of the volume. Again, this is the usable size of the volume rather than the actual size taken by the volume on the disk.

VxVM Volumes

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