As a fully unified storage array, Unity offers both block and file provisioning in the same enclosure. Disks are provisioned into storage pools that can host both block and file data. Connectivity is offered for both block and file protocols.

• Block connectivityUse iSCSI or Fibre Channel to access LUNs, consistency groups, VMware datastores, and VMware virtual volumes.
• File connectivityNetwork-attached storage (NAS) servers can host
  file systems that are accessed using server message block (SMB) shares
  or network file system (NFS) shares. NAS servers can also host VMware NFS datastores. 

Traditional storage pools

All storage resources are provisioned from storage pools. A storage pool is a collection of physical disks that are arranged into an aggregate group, with some form of RAID applied to the disks to provide redundancy. Traditional storage pools apply RAID protection to discrete groups of drives within the storage pool.

On a hybrid system, storage pools can consist of disks of varying types. These disks are sorted into one of three tiers: extreme performance (flash), performance (SAS), and capacity (NL-SAS). For hybrid systems with a fully automated storage tiering for virtual pools (FASTTM VP) license, you can configure a storage pool to contain multiple tiers of disks. This is known as a heterogeneous storage pool. When combined with FAST VP, heterogeneous storage pools can provide efficient balancing of data between tiers without requiring user intervention. Storage pools can also contain a single tier of disks; this is known as a homogeneous storage pool.

Figure 3 shows the different storage pool configurations.

Figure 3.        Storage pool configurations

Each tier in a storage pool can have a different RAID configuration set. Another consideration for storage pools is the “hot spare policy.” The Unity system reserves one spare drive per 31 drives to serve as a spare for the system. A spare drive can replace a faulted drive in a storage pool if it matches the drive type of the faulted disk. Any unbound drive can serve as a spare, but Unity enforces the “1 per 31” rule.

Note: Dell EMC recommends that a storage pool always has at least 10 percent free capacity to maintain proper functioning.

Dell EMC recommends using fewer storage pools within Unity systems to reduce complexity and increase flexibility. It might be appropriate to configure multiple storage pools to:

• Separate workloads with different I/O profiles
• Separate pools where FAST cache is and is not active
• Dedicate resources to meet specific performance goals
• Separate resources for multitenancy

All-flash pools

All-flash pools provide the highest level of performance in Unity systems. Use an all-flash
pool when the application requires the highest storage performance at the lowest response time. SAS Flash 3 or SAS Flash 4 drives are preferred for all-flash pools, as they provide higher capacities and lower cost per GB.

Dell EMC recommends using only a single drive size and a single RAID width within an all-flash pool. You can use 1.6 TB SAS Flash 3 or SAS Flash 4 drives and configure them all with RAID 5 8+1, for example.

Hybrid pools

Hybrid pools contain more than one type of drive technology. In addition to flash drives, hybrid pools can contain SAS or near-line SAS (NL-SAS) drives. Hybrid pools typically provide greater capacity at a lower cost than all-flash pools, but with lower overall performance and higher response times. Use hybrid pools for applications that do not require consistently low response times, or that have large amounts of mostly inactive data.

Note: Dell EMC recommends provisioning a flash tier in hybrid pools. The minimum recommended flash capacity is approximately five percent of the pool capacity. For more information, see the Dell EMC Unity: Best Practices Guide.

Dynamic pools

In Unity All Flash models running OE version 4.2.x or later, all new pools created in the Unisphere GUI are dynamic pools. New pools created in the Unisphere CLI and REST API are dynamic pools by default.

Note: Dynamic pools are only supported on Unity All Flash models with Unity OE version 4.2 and later. For Unity All Flash models running OE version 4.2.x or later, you can create traditional pools using the Unisphere CLI or REST API.

Dynamic pools implement advanced RAID technology. When creating a dynamic pool in Unisphere, the user selects the required RAID type. The RAID width is automatically selected by the system and depends on the number of drives selected at the time of pool creation.

Table 3 shows the relationship between the RAID type, the RAID width, and the minimum number of drives required to create the pool. Table 3 shows only the smallest RAID widths supported and the minimum number of drives required to create them. For  example, the smallest RAID 5 RAID width that the system supports is a 4+1. The smallest dynamic pool created with the RAID 5 4+1 configuration is six drives. The minimum drive count includes the number of drives specified in the RAID width, plus an extra drive to satisfy the spare space requirements.

Table 3.           RAID requirements for dynamic pools

Note: For more information, see the Dell EMC Unity: Dynamic Pools Overview.

Unlike traditional pools, dynamic pools do not require dedicated hot spare drives. Instead, space within each pool is reserved and can only be used to replace a failing or failed drive. After a dynamic pool is created and the drives are partitioned into drive extents, a number of drive extents are reserved and marked as spare space extents.

Dynamic pools have the following advantages over traditional pools:

• Drives are not wasted, because there are no fixed spares. All drives in the system can be added to a pool. This prolongs the life of the drives in the pool, because the load is spread across additional drives.
• Rebuild times are usually much faster than with traditional pools. Because spare capacity for a dynamic pool is spread across multiple drives rather than concentrated in on a single hot spare drive, more drives contribute to the rebuilding process when a drive fails.
• Pools can usually be expanded based on the capacity required. For example, you can add one drive at a time to a dynamic pool, providing provisioning flexibility and cost savings.

RAID protection

Unity applies RAID protection to the drives in a storage pool to protect user data against drive failures. The available RAID types are:

• RAID 1/0―Provides the highest level of performance from a given set of drive resources, with the lowest CPU requirements. Only 50 percent of the total drive capacity is usable. This RAID type is best suited for applications with fast or high processing requirements.
• RAID 5―Provides the best usable capacity from a set of drive resources, but with lower overall performance and availability than RAID 1/0. RAID 5 is best suited for transaction processing.
• RAID 6―Provides better availability than RAID-5 and better usable capacity than RAID 1/0, but has the lowest performance potential of the three RAID types. It is best suited for read-biased workloads, such as archiving and backup to drive.

FAST VP

FAST VP is an automated feature that optimizes disk utilization. It ensures that your data is distributed effectively across the various media types in a multi-tier pool. FAST VP defines three tiers of drive types:

• Extreme performance tierFlash disks
• Performance tierSerial-attached SCSI (SAS) disks
• Capacity tierNL-SAS disks

When a storage pool with multiple tiers is created on a hybrid Unity system with licensed FAST VP, block and file resources created on that storage pool are subject to FAST VP policies. FAST VP policies can specify where to place the initial data populated to a storage resourcethe highest tier or the lowest tier. As the data is manipulated, FAST VP relocates data across the tiers based on usage, as shown in Figure 4.

Figure 4.        FAST VP relocation

Data that is accessed frequently is placed on the best-performing tier, while less-frequently accessed data is moved to the high-capacity tiers. Data relocation is at a 256 MB granularity and can be a scheduled recurring event. Combined with FAST cache and the other multicore optimizations, FAST VP ensures that your disks are used efficiently.

Table 4 describes the FAST VP tiering policy settings, which are defined at the data-resource level. This policy defines both the initial tier placement and the ongoing automated tiering of data during data relocation operations. When creating a FAST VP-enabled storage pool, use the highest available tier for the data volumes on flash and auto tier for the log volumes on SAS.

Table 4.           Tiering policy information