Array configurations

An array is an ordered group of physical devices (drives) that is used to define logical volumes or devices. An array is a type of MDisk that is made up of drives; these drives are members of the array. A Redundant Array of Independent Disks (RAID) is a method of configuring member drives to create high availability and high-performance systems.

The system supports distributed RAID array configurations. Distributed RAID array that contains more than two drives have a minimum of one rebuild area. All the drives in an array can process I/O data and provide faster rebuild times when a drive fails. The RAID level provides different degrees of redundancy and performance; it also determines the minimum and maximum number of member drives in the array.

In order to restore redundancy for degraded distributed RAID arrays, the rebuild-in-place process is used for reconstructing the data (or parity) directly back into the replaced member drive. Degraded distributed RAID arrays with only two member drives use the rebuild-in-place process to restore redundancy, copying the data directly back into the replaced member drive.

If encryption is enabled on your system, you can create RAID arrays that are encrypted. For more information, see Configuring encryption.

Distributed RAID arrays

Distributed RAID array configurations typically contain rebuild areas that are used to maintain redundancy after a drive failure. RAID array configurations that do not contain rebuild areas still maintain redundancy after a drive failure, by using rebuild-in-place. Data is rebuilt and redundancy is restored much more rapidly.

For more information and details about distributed RAID arrays, see Distributed RAID array properties.

You can expand distributed RAID arrays to increase the available capacity of the array or add usable capacity. As part of the expansion, the system automatically migrates data for optimal performance for the new expanded configuration. Expansion of distributed RAID arrays supports incremental growth of available capacity for arrays and is compatible with other functions, such as IBM® Easy Tier® and data migrations.

The type of RAID array configuration that the system uses is based on the type of drives that are detected through the serial-attached SCSI (SAS) or Non-Volatile Memory express (NVMe) protocols. If your system contains SAS-attached drives only, the system can configure distributed RAID arrays based on the number of drives available and the characteristics of those drives. Distributed RAID arrays are preferred because distributed RAID arrays provide better performance and rebuild times. For systems that support only NVMe drives, only distributed RAID arrays are created for array configuration since these drives have improved performance for high-demand drives. Distributed RAID arrays are also supported for compressing drives. NVMe drives are only supported within certain models of control enclosures. For systems that contain both NVMe and SAS drives, the system restricts mixing these different types of drives in the same RAID array configuration. Drive members within the array must be as similar as possible to avoid performance problems and issues replacing drives.

The system also supports NVMe drives that are self-compressing, where the drive itself compresses data as it is written to the drive. If the array includes self-compressing drives, the system uses the following rules when the array is created:
  • Non-compressing drives and self-compressing drives cannot be in the same RAID array configuration.
  • All drives in the array have the same usable capacity and use the same compression algorithm.
  • If the array contains at least 7 drives, the system recommends a distributed RAID 6 array, which is preferred to ensure capacity for self-compressing drives during rebuild operations.
  • Only one distributed RAID array of self-compressing drives is allowed per storage pool.

Distributed RAID array configuration guidelines

The management GUI automatically defaults to the recommended number of rebuild areas that is based on the current width and other settings for the distributed RAID array. The width of the array includes the number of physical drives and rebuild areas that are needed to ensure redundancy if drives fail. The system can recommend a maximum of four rebuild areas for distributed RAID arrays for SAS-attached drives and non-compressing NVMe distributed RAID arrays. Compressing NVMe distributed RAID arrays support one rebuild area. Other settings, such as drive class, are also used to determine the best configuration for arrays. For example, if you are adding storage or expanding a distributed RAID array, the management GUI disables any drive classes that are incompatible with drives already in the pool. It then analyzes the current array configuration in the pool to recommend stripe widths and drive counts.

After system setup, you must configure storage by creating pools and assigning storage to specific pools. Ensure that a pool or pools are created before you assign storage. In the management GUI, select Pools > Actions > Add Storage. The Add Storage selection automatically configures existing drives into arrays based on array characteristics.

Use the lsarrayrecommendation command to display the system recommendations for configuring an array. However, the command makes no recommendation for distributed RAID arrays if the storage pool already contains one or more distributed RAID arrays that are made of compressing drives. Also, the command makes no recommendation if a pool extent size is less than the recommended extent size. To configure a distributed RAID array, you can use the mkdistributedarray command. For more information on arrays commands, refer to the Array commands. The mkdistributedarray command does not support the creation of a second distributed RAID array that is made of any compressing drives, if the storage pool already contains one or more distributed RAID arrays made of compressing drives.

The management GUI does not provide array recommendation if the following conditions are present:
  • The selected pool already contains a distributed RAID array with compressing drives.
  • The selected drive class might not be compatible with other drives that are already in the pool.
  • The number of selected drives is not enough to create a valid RAID array.
  • The extent size is smaller than the minimum extent size.
If these conditions are present, you can take one of the following actions:
  • Select a different drive class.
  • Select a different storage pool.
  • Select a different number of candidate drives.
  • Expand an existing array in the pool instead of adding a new array. For more information, see Expanding an MDisk in a distributed array.

For more information on supported array types and RAID levels, see Configuration limits.