Virtual disks (VDisks)

A VDisk is a logical disk that the cluster presents to the storage area network (SAN). Application servers on the SAN access VDisks, not managed disks (MDisks). VDisks are created from a set of extents in an MDisk group. There are three types of VDisks: striped, sequential, and image.

Types

You can create the following types of VDisks:

Striped
The striping is at extent level. One extent is allocated, in turn, from each managed disk that is in the group. For example, a managed disk group that has 10 MDisks takes one extent from each managed disk. The 11th extent is taken from the first managed disk, and so on. This procedure, known as a round-robin, is similar to RAID-0 striping.
Attention: Attention: Care should be taken when specifying a stripe set if your MDisk group contains MDisks of unequal size. By default, striped VDisks are striped across all MDisks in the group. If some of the MDisks are smaller than others, the extents on the smaller MDisks will be used up before the larger MDisks run out of extents. Manually specifying the stripe set in this case, might result in the VDisk not being created.

You can also supply a list of MDisks to use as the stripe set. This list can contain two or more MDisks from the managed disk group. The round-robin procedure is used across the specified stripe set.

The following figure shows an example of a managed disk group containing three MDisks. This figure also shows a striped virtual disk created from the extents available in the group.

Figure 5. Managed disk groups and VDisks
Managed disk groups and VDisks
Sequential
When selected, extents are allocated sequentially on one managed disk to create the virtual disk if enough consecutive free extents are available on the chosen managed disk.
Image
Image-mode VDisks are special VDisks that have a direct relationship with one managed disk. If you have a managed disk that contains data that you want to merge into the cluster, you can create an image-mode virtual disk. When you create an image-mode virtual disk, a direct mapping is made between extents that are on the managed disk and extents that are on the virtual disk. The managed disk is not virtualized. In other words, the logical block address (LBA) x on the managed disk is the same as LBA x on the virtual disk.

When you create an image-mode VDisk, you must assign it to a managed disk group. An image-mode VDisk must be at least one extent in size. In other words, the minimum size of an image-mode VDisk is the extent size of the MDisk group to which it is assigned.

The extents are managed in the same way as other VDisks. When the extents have been created, you can move the data onto other MDisks that are in the group without losing access to the data. After you move one or more extents, the virtual disk becomes a real virtualized disk, and the mode of the managed disk changes from image to managed.

Attention: Attention: If you add an MDisk to an MDisk group as a managed disk, any data on the MDisk will be lost. Ensure that you create image-mode VDisks from the MDisks that contain data before you start adding any MDisks to groups.

MDisks that contain existing data have an initial mode of unmanaged, and the cluster cannot determine whether they contain partitions or data.

The status of a virtual disk consists of three settings. The following table describes the different states of a virtual disk:

Table 4. Virtual disk status
Status Description
Online The virtual disk is online and available if both nodes in the I/O group can access the virtual disk. A single node will only be able to access a VDisk if it can access all the MDisks in the MDisk group associated with the VDisk.
Offline The VDisk is offline and unavailable if both nodes in the I/O group are missing or none of the nodes in the I/O group that are present can access the VDisk.
Degraded The status of the virtual disk is degraded if one node in the I/O group is online and the other node is either missing or cannot access the virtual disk.

You can also use more sophisticated extent allocation policies to create VDisks. When you create a striped virtual disk, you can specify the same managed disk more than once in the list of MDisks that are used as the stripe set. This is useful if you have a managed disk group in which not all the MDisks are of the same capacity. For example, if you have a managed disk group that has two 18 GB MDisks and two 36 GB MDisks, you can create a striped virtual disk by specifying each of the 36 GB MDisks twice in the stripe set so that two thirds of the storage is allocated from the 36 GB disks.

If you delete a virtual disk, you destroy access to the data that is on the virtual disk. The extents that were used in the virtual disk are returned to the pool of free extents that is in the managed disk group. The deletion might fail if the virtual disk is still mapped to hosts. The deletion might also fail if the virtual disk is still part of a FlashCopy or a Remote Copy mapping. If the deletion fails, you can specify the force-delete flag to delete both the virtual disk and the associated mappings to hosts. Forcing the deletion will also delete the copy services relationship and mappings.

(C) Copyright International Business Machines Corporation 2004