Storage pools
Use Pools page to configure and manage storage pools, internal and external storage, MDisks, and to migrate existing storage to the system. In general, a pool or storage pool is an allocated amount of capacity that jointly contains all of the data for a specified set of volumes. The system supports standard pools (parent pools and child pools) and data reduction pools.
Parent Pools
Parent pools receive their usable capacity from MDisks. All MDisks in a pool are split into extents of the same size. Volumes are created from the extents that are available in the pool. You can add MDisks to a pool at any time either to increase the number of extents that are available for new volume copies or to expand existing volume copies. The system automatically balances volume extents between the MDisks to provide the best performance to the volumes.
To track the space that is available on an MDisk, the system divides each MDisk into chunks of equal size. These chunks are called extents and are indexed internally. Extent sizes can be 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, or 8192 MB. The choice of extent size affects the total amount of storage that is managed by the system.
You specify the extent size when you create a new parent pool. You cannot change the extent size later; it must remain constant throughout the lifetime of the parent pool.
You cannot use the data migration function to migrate volumes between parent pools that have different extent sizes. However, you can use volume mirroring to move data to a parent pool that has a different extent size.
Use volume mirroring to add a copy of the disk from the destination pool. After the copies are synchronized, you can free up extents by deleting the copy of the data in the source pool. The FlashCopy® function and Metro Mirror can also be used to create a copy of a volume in a different pool.
A system can manage 2^22 extents. For example, with a 16 MB extent size, the system can manage up to 16 MB x 4,194,304 = 64 TB of storage.
When you choose an extent size, consider your future needs. For example, if you currently have 40 TB of storage and you specify an extent size of 16 MB for all parent pools, the capacity of the system is limited to 64 TB of storage in the future. If you select an extent size of 64 MB for all parent pools, the capacity of the system can grow to 256 TB.
Using a larger extent size can waste storage. When a volume is created, the storage capacity for the volume is rounded to a whole number of extents. If you configure the system to have many small volumes and you use a large extent size, storage can be wasted at the end of each volume.
When you create or manage a parent pool, consider the following general guidelines:
Child Pools
Instead of being created directly from MDisks, child pools are created from existing usable capacity that is assigned to a parent pool. As with parent pools, volumes can be created that specifically use the usable capacity that is assigned to the child pool. Child pools are similar to parent pools with similar properties and can be used for volume copy operation.
When the child pool is created, the usable capacity for a child pool is reserved from the usable capacity of the parent pool. The usable capacity for the child pool must be smaller than the usable capacity in the parent pool. After the child pool is created, the amount of usable capacity that is specified for the child pool is no longer reported as usable capacity of its parent pool.
Child pools can be assigned to an ownership group. An ownership group defines a subset of users and objects within the system. You can create ownership groups to further restrict access to specific resources that are defined in the ownership group. Only users with Security Administrator roles can configure and manage ownership groups.
Ownership can be defined explicitly or it can be inherited from the user, user group, or from other parent resources, depending on the type of resource. Ownership of child pools must be assigned explicitly, and they do not inherit ownership from other parent resources. New or existing volumes that are defined in the child pool inherit the ownership group that is assigned for the child pool.
Data Reduction Pools
To use data reduction technologies on the system, you need to create a data reduction pool, create volumes with the data reduction pool, and map these volumes to hosts that support SCSI unmap commands.
Data reduction is a set of techniques that can be used to reduce the amount of usable capacity that is required to store data. An example of data reduction includes data deduplication. Data reduction can increase storage efficiency and performance and reduce storage costs, especially for flash storage. Data reduction reduces the amount of data that is stored on external storage systems and internal drives by reclaiming previously used capacity that are no longer needed by host systems. To estimate potential capacity savings that data reduction can provide on the system, use the Data Reduction Estimation Tool (DRET). This tool analyzes existing user workloads that are being migrated to a new system. The tool scans target workloads on all attached storage arrays, consolidates these results, and generates an estimate of potential data reduction savings for the entire system.
For more information about DRET, see https://www.ibm.com/support/pages/node/6217841. For more information about Comprestimator, see https://www.ibm.com/support/pages/node/6209688.
The system supports data reduction pools which can use different capacity savings methods simultaneously, increasing the capacity savings across the entire pool. Data reduction pools also support deduplication. When deduplication is specified for a volume, duplicate versions of data are eliminated and not written to storage, thus saving more usable capacity. Some models or software versions require specific hardware or software to use this function. For more information, see planning data reduction pools and deduplication.
Support for the host SCSI unmap command is enabled by default.
Verify whether the storage system supports data reduction technologies, like data deduplication. If you use storage systems that support data reduction technologies, you can also configure data reduction on the storage systems. The storage system can reclaim that freed storage and reorganize the data on other volumes to more efficiently use the capacity. For standard-provisioned volumes, the system fully controls storage on these storage systems. When a volume is deleted, capacity is freed on the system and can be reallocated; the storage system is not aware of this freed space. However, if the storage system uses compression, thin-provisioning, or deduplication, the storage system controls the use of the usable capacity. In this configuration, when capacity is freed, the system notifies the storage system that capacity is no longer needed. The storage system can then reuse that capacity or free it as reclaimable capacity. The system also supports reclaimable capacity from certain internal drives, such as the 15 TB tier 1 flash drives, which can improve performance on these types of drives.
Extent size (in gigabytes) | Overhead capacity requirements (in terabytes)1 |
---|---|
1 GB or smaller | 1.1 TB |
2 GB | 2.1 TB |
4 GB | 4.2 TB |
8 GB | 8.5 TB |
Pool states
State | Description |
---|---|
Online | The pool is online and available. All the MDisks in the pool are available. |
Degraded paths | This state indicates that one or more nodes in the system cannot access all
the MDisks in the pool. A degraded path state is most likely the result of
incorrect configuration of either the storage system or the Fibre Channel fabric.
However, hardware failures in the storage system, Fibre Channel fabric, or node
might also be a contributing factor to this state. To recover from this state, follow these
steps: |
Degraded ports | This state indicates that one or more 1220 errors were logged against the MDisks in the pool. The 1220 error indicates that the remote Fibre Channel port was excluded from the MDisk. This error might cause reduced performance on the storage system and usually indicates a hardware problem with the storage system. To fix this problem, you must resolve any hardware problems on the storage system and fix the 1220 errors in the event log. To resolve these errors in the log, click in the management GUI. This action displays a list of unfixed errors that are currently in the event log. For these unfixed errors, select the error name to begin a guided maintenance procedure to resolve them. Errors are listed in descending order with the highest priority error listed first. Resolve highest priority errors first. |
Offline | The pool is offline and unavailable. No nodes in the system can access the
MDisks. The most likely cause is that one or more MDisks are offline or excluded. Attention: If a single MDisk in a pool is offline and cannot be seen by
any of the online nodes in the system, the pool of which this MDisk is a member goes offline.
This causes
all of the volume copies that are being presented by this pool to go offline. Take care when you
create pools to ensure an optimal configuration.
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