PRIMECLUSTER Global Disk Services Configuration and Administration Guide 4.1 (Linux)

2.7.2.4 Shadow Volume

A volume created in a shadow group or a shadow disk of the single type is called a shadow volume. The users and applications will access data on the shadow volumes instead of the physical disks. A shadow volume is the shadow object corresponding to a logical volume that is an SDX object.

When logical volumes and shadow volumes do not have to be classified particularly, they may be called "volumes" collectively.

Create shadow volumes conforming to the following rules in order to use the shadow volumes to access data in the corresponding logical volumes.

• Equal to the corresponding logical volumes in size. For volume sizes, check the BLOCKS field displayed with the sdxinfo -V command.
• Whose the first block numbers must be consistent with the first block numbers of the corresponding logical volumes. Therefore, create shadow volumes within the same shadow group or shadow disk in ascending order in conformity to the order of the first block numbers numbers of the corresponding logical volumes. For the first block numbers of volumes, check the 1STBLK field displayed with the sdxinfo -V command.

Synchronization copying is not conducted when a shadow volume of the mirror type is created. When a shadow volume corresponding to a mirror volume is to be created, synchronization of the mirror volume must be ensured in advance by the use of GDS managing that mirror volume.

Shadow volumes and the corresponding logical volumes are managed independently. For example, the change on the slice status in one volume is not updated on the slice status in the other volume. For this reason, you must note certain operational particulars when using shadow volumes. For details, see "Shadow Volume."

 

Attributes

A shadow volume has the following attributes.

Name

This attribute identifies the shadow volume within a shadow class.

JRM

This attribute identifies the just resynchronization mechanism mode. The following value is set.

off

JRM is inactive.

Lock Volume

This attribute identifies the "Lock volume" mode. The following value is set.

off

The volume will not be locked.

Access Mode

This attribute sets the default access mode. If a volume is activated without specifying the access mode, the default setting will be applied. The following value is set.

ro

The default is set to read only.

The access mode attribute value cannot be set to rw (read and write). To write data on a shadow volume, the shadow volume must first be stopped and reactivated in the read and write mode using the sdxshadowvolume -N command with the -e mode=rw option.

Physical Slice

This attribute is always set to off regardless whether the shadow volume has a physical slice, which means the shadow slice is registered with partition table.

 

Operation

The operation other than status display from Management View is unsupported. Use the command for each operation.

Create

A shadow volume can be created in the highest level shadow group or a shadow disk of the single type with the sdxshadowvolume -M command.

Delete

The shadow volume can be deleted with the sdxshadowvolume -R command.

Start

The shadow volume can be started with the sdxshadowvolume -N command.

Stop

The shadow volume can be stopped with the sdxshadowvolume -F command.

Status Display

The volume status can be displayed with the sdxinfo command.

For details about GDS Management View, see "Viewing Configurations and Monitoring Statuses."

 

Status

A shadow volume can have the following status.

ACTIVE

Data is accessible.

After a shadow volume is started normally, it is given ACTIVE status. Here, there are one or more shadow slices in the ACTIVE status in the shadow volume.

STOP

Data is inaccessible, but the shadow volume can be activated and be ACTIVE.

After the shadow volume is stopped normally, it is given STOP status. Here, there are one or more shadow slices in STOP status in the shadow volume.

 

Reference

There are the following points of concern for shadow volumes.

Rules

Number of Volumes

Important Point

Volume Size

Volume Access Mode

Shadow Volume

 

Multiple physical disks virtualized as a logical volume in a certain domain (domain alpha in the figure below) can be virtualized as a shadow volume in another domain (domain beta in the figure below), and the shadow volume can be used in order to access the data on the logical volume in domain alpha from domain beta. The primary service (service A in the figure below) can be run by the use of the logical volume in domain alpha, and another service (service B: for example, backup and restore, restore, batch processing) can be run by the use of the shadow volume in domain beta. However, service A and service B cannot be run simultaneously. If they are run simultaneously, data consistency between disks is not ensured.

[Figure: A Logical Volume and a Shadow Volume]

 

One physical disk temporarily detached from mirroring among multiple physical disks virtualized as a mirror volume in a certain domain (domain alpha in the figure below) can be virtualized as a shadow volume in another domain (domain beta in the figure below), and the shadow volume can be used in order to access the data on the temporarily detached slice in domain alpha from domain beta. The primary service (service A in the figure below) can be run by the use of the mirror volume in domain alpha from which one slice is temporarily detached, and another service (service C: for example, backup and restore, restore, batch processing) can be run by the use of the shadow volume in domain beta simultaneously. However, when a different service (service B in the figure below) is run by the use of the temporarily detached slice in domain alpha, service B and service C cannot be run simultaneously. If they are run simultaneously, data consistency between disks is not ensured.

 

[ Figure: Mirror Slice Temporary Detachment and a Shadow Volume]

Multiple physical disks virtualized as a proxy volume in a certain domain (domain alpha in the figure below) can be virtualized as a shadow volume in another domain (domain beta in the figure below), and the shadow volume can be used in order to access the data on the proxy volume in domain alpha from domain beta. The primary service (service A in the figure below) can be run by the use of the master volume in domain alpha from which the proxy is parted, and another service (service C: for example, backup and restore, restore, batch processing) can be run by the use of the shadow volume in domain beta simultaneously. However, when a different service (service B in the figure below) is run by the use of the proxy volume in domain alpha, service B and service C cannot be run simultaneously. If they are run simultaneously, data consistency between disks is not ensured.

 

[Figure: A Proxy Volume and a Shadow Volume]