A virtual disk (VDisk) is a logical disk that the cluster presents to the hosts.
To keep a VDisk accessible even when a managed disk on which it depends has become unavailable, a mirrored copy can be added to a selected VDisk. Each VDisk can have a maximum of two copies. Each VDisk copy is created from a set of extents in an MDisk group.
Application servers on the SAN access VDisks, not managed disks (MDisks).
There are three types of VDisks: striped, sequential, and image.
Types
- Striped
- A VDisk copy that has been striped is at the extent level. One
extent is allocated, in turn, from each MDisk that is in the group.
For example, an MDisk group that has 10 MDisks takes one extent from
each MDisk. The 11th extent is taken from the first MDisk, and so
on. This procedure, known as a round-robin, is similar to RAID-0 striping.
You can also supply a list of MDisks to use as the stripe set. This list can contain two or more MDisks from the MDisk group. The round-robin procedure is used across the specified stripe set.
Attention: By default, striped VDisk copies are striped across all MDisks in the group. If some of the MDisks are smaller than others, the extents on the smaller MDisks are used up before the larger MDisks run out of extents. Manually specifying the stripe set in this case might result in the VDisk copy not being created.If you are unsure if there is sufficient free space to create a striped VDisk copy, select one of the following options:- Check the free space on each MDisk in the group using the svcinfo lsfreeextents command.
- Let the system automatically create the VDisk copy by not supplying a specific stripe set.
Figure 1 shows an example of an MDisk group that contains three MDisks. This figure also shows a striped VDisk copy that is created from the extents that are available in the group.
- Sequential
- When extents are selected, they are allocated sequentially on one MDisk to create the VDisk copy if enough consecutive free extents are available on the chosen MDisk.
- Image
- Image-mode VDisks are special VDisks that have a direct relationship
with one MDisk. If you have an MDisk that contains data that you want
to merge into the cluster, you can create an image-mode VDisk. When
you create an image-mode VDisk, a direct mapping is made between extents
that are on the MDisk and extents that are on the VDisk. The MDisk
is not virtualized. The logical block address (LBA) x on
the MDisk is the same as LBA x on the VDisk.
When you create an image-mode VDisk copy, you must assign it to an MDisk group. An image-mode VDisk copy must be at least one extent in size. The minimum size of an image-mode VDisk copy is the extent size of the MDisk group to which it is assigned.
The extents are managed in the same way as other VDisk copies. 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 VDisk copy becomes a virtualized disk, and the mode of the MDisk changes from image to managed.
Attention: If you add a managed mode MDisk to an MDisk group, any data on the MDisk is 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 if it contains partitions or data.
You can use more sophisticated extent allocation policies to create VDisk copies. When you create a striped VDisk, you can specify the same MDisk more than once in the list of MDisks that are used as the stripe set. This is useful if you have an MDisk group in which not all the MDisks are of the same capacity. For example, if you have an MDisk group that has two 18 GB MDisks and two 36 GB MDisks, you can create a striped VDisk copy 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 VDisk, you destroy access to the data that is on the VDisk. The extents that were used in the VDisk are returned to the pool of free extents that is in the MDisk group. The deletion might fail if the VDisk is still mapped to hosts. The deletion might also fail if the VDisk is still part of a FlashCopy®, Metro Mirror or Global Mirror mapping. If the deletion fails, you can specify the force-delete flag to delete both the VDisk and the associated mappings to hosts. Forcing the deletion deletes the Copy Services relationship and mappings.
States
A VDisk can be in one of three states: online, offline, and degraded. Table 1 describes the different states of a VDisk.| State | Description |
|---|---|
| Online | At least one synchronized copy of the VDisk is online and available if both nodes in the I/O group can access the VDisk. A single node can only access a VDisk if it can access all the MDisks in the MDisk group that are 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 any synchronized copy of the VDisk. The VDisk can also be offline if the VDisk is the secondary of a Metro Mirror or Global Mirror relationship that is not synchronized. A space-efficient VDisk goes offline if a user attempts to write an amount of data that exceeds the available disk space. |
| Degraded | The status of the VDisk is degraded if one node
in the I/O group is online and the other node is either missing or
cannot access any synchronized copy of the VDisk. Note: If you have
a degraded VDisk and all of the associated nodes and MDisks are online,
call the IBM® Support
Center for
assistance.
|
Cache modes
You can select to have read and write operations stored in cache by specifying a cache mode. You must specify the cache mode when you create the VDisk. After the VDisk is created, you cannot change the cache mode.
Table 2 describes the two types of cache modes for a VDisk.