Volumes
A volume is a logical disk that the system presents to attached hosts.
Application servers access volumes, not MDisks, or, drives. To keep a volume accessible even when an MDisk on which it depends becomes unavailable, a mirrored copy can be added to a selected volume. Each volume copy is created from a set of extents in a pool.
You can create different types of volumes, depending on the type of topology that is supported and configured on your system. All systems support standard topology, which is a single-site configuration. For systems with single-site configuration, you can create basic, mirrored, or custom volumes. The system supports several capacity savings methods, such as thin-provisioning and compression. Thin provisioning creates a volume with more virtual than provisioned capacity that allows the capacity to grow as it is needed. With compressed volumes, data is compressed as it is written to the volume, which saves capacity on the volume. Deduplication can be configured with volumes that use different capacity saving methods, such as thin-provisioning. If you select to deduplicate volume data in a multi-site topology, the volume copies must be in data reduction pools on both sites.
Volumes can be assigned to an ownership group. An ownership group defines a subset of users and objects within the system.
- The volume inherits the ownership group of the child pools that provide capacity for the volume and its copies. Volume copies can be created in different ownership groups for backup up scenarios. However, this value must be set intentionally by users that are not defined in ownership groups. When you create a volume copy or migrate volumes to other pools, you can specify child pools that are defined in different ownership groups in the management GUI, which establishes inconsistent ownership. When you create a volume copy or migrating a volume in the command-line interface, use the -inconsistentownershipgroup to allow for inconsistent ownership groups. However, it is not recommended to leave volumes or volume copies in different ownership groups. After the migration, the user with Security Administrator role needs to ensure all volumes or copies are within the same ownership group as the users who need access.
- With volume groups, the volume group and its volumes can belong to different ownership groups. However, the ownership of a volume group does not impact the ownership of the volumes that it contains.
Types
- A volume copy that was striped is at the extent level. One extent is allocated, in turn, from
each MDisk that is in the storage pool. For example, a storage pool that has 10 MDisks takes one
extent from each MDisk. The 11th extent is taken from the first MDisk, and so on. This procedure,
which is 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 storage pool. The round-robin procedure is used across the specified stripe set.
Attention: By default, striped volume copies are striped across all MDisks in the storage pool. 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 volume copy not being created.If you are unsure if sufficient available capacity is available to create a striped volume copy, select one of the following options:This figure shows an example of a storage pool that contains three MDisks. This figure also shows a striped volume copy that is created from the extents that are available in the storage pool.
Figure 1. Storage pools and volumes 
- When extents are selected, they are allocated sequentially on one MDisk to create the volume copy if enough consecutive free extents are available on the chosen MDisk.
- Image-mode volumes are special volumes that have a direct relationship with one MDisk. If you
have an MDisk that contains data that you want to merge into the clustered system, you can
create an image-mode volume. When you create an image-mode volume, a direct mapping is made between
extents that are on the MDisk and extents that are on the volume. The MDisk is not virtualized. The
logical block address (LBA) x on the MDisk is the same as LBA
x on the volume.
When you create an image-mode volume copy, you must assign it to a storage pool. An image-mode volume copy must be at least one extent in size. The minimum size of an image-mode volume copy is the extent size of the storage pool to which it is assigned.
The extents are managed in the same way as other volume copies. When the extents are created, you can move the data onto other MDisks that are in the storage pool without losing access to the data. After you move one or more extents, the volume 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 a storage pool, any data on the MDisk is lost. Ensure that you create image-mode volumes from the MDisks that contain data before you start adding any MDisks to storage pools.MDisks that contain existing data have an initial mode of unmanaged, and the clustered system cannot determine whether it contains partitions or data.
Image mode MDisks are members of a storage pool, but they do not contribute to free extents. Image mode volumes are not affected by the state of the storage pool because the storage pool controls image mode volumes through the association of the volume to an MDisk. Therefore, if an MDisk that is associated with an image mode volume is online and the storage pool of which they are members goes offline, the image mode volume remains online. Conversely, the state of a storage pool is not affected by the state of the image mode volumes in the storage pool.
Image mode volumes can be used to migrate existing data from an external storage system. However, they cannot be used to export the replicated volumes to an external storage system. To configure replication on image mode volumes, add volumes to a volume group and then assign a replication policy. Image mode volumes cannot be added to volume groups in a storage partition. A volume group that contains volume with a cache mode other than readwrite, the replication direction cannot be changed.
To maintain replication on an external storage system until a new recovery copy is established, perform the following:- Create an image mode volume with the cache mode set to none.
- Once the new recovery copy is established, change the cache mode to readwrite.
You can use more sophisticated extent allocation policies to create volume copies. When you create a striped volume, you can specify the same MDisk more than once in the list of MDisks that are used as the stripe set. This allocation is useful if you have a storage pool in which not all the MDisks are of the same capacity. For example, if you have a storage pool that has two 18 GB MDisks and two 36 GB MDisks, you can create a striped volume copy by specifying each of the 36 GB MDisks twice in the stripe set so that two-thirds of the storage are allocated from the 36 GB disks.
If you delete a volume, you destroy access to the data that is on the volume. The extents that were used in the volume are returned to the pool of free extents that is in the storage pool. The deletion might fail if the volume is still mapped to hosts. If the deletion fails, you can specify the force-delete flag to delete both the volume and the associated mappings to hosts. Forcing the deletion deletes the Copy Services relationship and mappings.
States
This table describes the different possible states of a volume.
| State | Description |
|---|---|
| Online | At least one synchronized copy of the volume is online and available if both nodes in the I/O group can access the volume. A single node can access a volume only if it can access all the MDisks in the storage pool that are associated with the volume. |
| Offline | The volume is offline and unavailable if both nodes in the I/O group are missing, or if none of the nodes in the I/O group that are present can access any synchronized copy of the volume. A thin-provisioned volume goes offline if a user attempts to write an amount of data that exceeds the available disk space. |
| Degraded | The status of the volume 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 volume.
Note: If
a volume is degraded and all of the associated nodes and MDisks are online, call your support center
for assistance.
|
| Deleting | For thin-provisioned or compressed volume copies in data reduction pools, the
deleting status indicates that copies are being deleted. All volume copies, including fully
allocated copies, are not accessible until the delete operation completes. In addition, several
operations cannot be started until all copies are deleted. The following commands are restricted if
one copy of a volume is in the process of being deleted:
|
Cache modes
You can select to have read and write operations that are stored in cache by specifying a cache mode. You can specify the cache mode when you create the volume. After the volume is created, you can change the cache mode.
This table describes the types of cache modes for a volume.
| Cache mode | Description |
|---|---|
| readwrite | All read and write I/O operations that are performed by the volume are stored
in cache. This is the default cache mode for all volumes. A
volume or volume copy that is created from a data reduction pool must have a cache mode of
readwrite. If you try to create a thin provisioned or compressed volume copy from a data reduction
pool and the volume cache mode is not readwrite, the operation fails. |
| readonly | All read I/O operations that are complete by the volume are stored in the cache. |
| none | All read and write I/O operations that are complete by the volume are not stored in cache. |