• Understanding RAID (physical deployments only)


    Understanding RAID (physical deployments only)

    About RAID

    Redundant Array of Independent Disks (RAID) is a method for providing high levels of storage reliability by arranging drives in groups, and dividing and replicating data among the drives in a group. You select drive types and RAID configurations (RAID types and stripe widths) when you create a pool. Once a pool is configured, you cannot change the RAID type of a tier. However, in hybrid models, you can add a new tier with a different RAID type. (All-Flash models have only one tier.)

    RAID usually has the characteristics of parity, striping, or both:

    • Parity provides redundancy for blocks of data on the drives. Depending on the RAID type, this provides the ability to continue to operate with the loss of one or more drives.
    • Striping provides a mechanism for processing data that allows the comprehensive read/write performance of a RAID group to exceed the performance of its component drives.

    In hybrid models that are licensed for FAST VP and that have unused drives of different types, you can configure multiple storage tiers for the pool and can specify a different RAID configuration for each tier.

    The system supports RAID 5, 6, and 1/0 (also called RAID 10).

    RAID in dynamic pools

    In a dynamic pool, drives are partitioned into drive extents. These extents are combined into RAID extents, which are spread across multiple drives. The number of drive extents within a RAID extent depends on the RAID type and width. For example, a RAID 5 (4+1) RAID extent contains five drive extents. For redundancy purposes, the system ensures that no RAID extent contains two drive extents from the same drive. Also, each drive extent can only be part of one RAID extent.

    A RAID group in a dynamic pool is a collection of RAID extents. The number and size of RAID groups in a dynamic pool can vary depending on the number of drives and how the pool was created and expanded. A RAID extent can only be part of one RAID group.

    The following illustration shows a RAID group with a RAID 5 (4 + 1) configuration. This configuration requires six drives to meet the requirement for spare space. In this illustration, each rectangle represents one drive extent, and each color represents one RAID extent:

    Figure 1. RAID 5 (4+1) group in a dynamic pool
    RAID 5 (4+1) group in a dynamic pool
    RAID in traditional pools

    In a traditional pool, a RAID group contains sets of drives with the same capacity and redundancy on which you create one or more storage resources. The system creates one or more RAID groups for the pool based on the specified configuration. For example, when you create a storage resource in a RAID 5 (4 + 1) group, data is distributed equally across the five drives in the RAID group. If you want to create a pool using more than five drives, you must do so in multiples of the selected RAID stripe width.

    The following illustration shows a RAID 5 (4 + 1) group, along with the required spare drive.

    Figure 2. RAID 5 (4 + 1) group plus one spare drive in a traditional pool
    RAID 5 (4 + 1) group plus one spare drive in a traditional pool

    RAID configurations

    Pool tiers are built using a set of one or more individual drive groups based on the tier's RAID type and stripe width. The RAID type determines the performance characteristics of each drive group. The stripe width determines the fault characteristics of each drive group.

    For example, a RAID 5 drive group can still operate with the loss of one drive (traditional pools) or it's equivalent (dynamic pools). A RAID 5 (4+1), 5 drive configuration has less risk of multiple drive faults than a RAID 5 (12+1), 13 drive configuration.

    The following table describes the supported RAID types for the intended storage usage:

    Table 1. Supported RAID levels
    RAID level
    RAID 1/0 (also called RAID 10)
    Best suited for applications with fast or high processing requirements, such as enterprise servers and moderate-sized database systems. Provides both high performance and reliability at medium cost, while providing lower capacity per drive.
    RAID 5
    Best suited for transaction processing and often used for general purpose storage, as well as for relational database and enterprise resource systems. Depending on the drives used, this RAID type can provide a fairly low cost per MB while still retaining redundancy.
    RAID 6
    Best suited for read-biased workloads, such as archiving and backup to drive.
    Mixed RAID configurations (traditional pools only)

    If FAST VP is installed on the system, you can create a pool with multiple storage tiers. Each tier can have its own RAID type. Only one RAID type can be used within a tier, but the tier can have different stripe configurations. For example, you can mix RAID 5 (4+1) and RAID 5 (8+1) in a tier.

    To do this:

    • Select the Maximum Capacity RAID configuration when you create the pool. This configuration might mix RAID types in the pool.
    • Expand the pool using a different stripe width than currently exists in the pool.

    RAID for dynamic pools in Unisphere

    Unisphere automatically assigns a RAID width to a dynamic pool based on the selected RAID type and the selected number of drives. The following table describes how the RAID width gets assigned:

    RAID level
    Number of drives
    RAID Width
    RAID 5
    6 to 9
    10 to 13
    14 or more
    RAID 6
    7 to 8
    9 to 10
    11 to 12
    13 to 14
    15 to 16
    17 or more
    RAID 1/0
    3 to 4
    5 to 6
    7 to 8
    9 or more

    Drive IOPS by RAID type

    Front-end application workloads translate into different back-end drive workloads based on the RAID type in use. For front-end reads, there is no impact by RAID type: 1 front-end read I/O equals 1 back-end read I/O.

    The following table shows the impact by RAID type for random front-end writes.

    Table 2. IOPS by RAID type for front-end writes
    RAID type
    IOPS per 1 front-end write I/O
    RAID 1/0
    2 back-end write I/0s
    RAID 5
    2 back-end reads and 2 back-end writes
    RAID 6
    3 back-end reads and 3 back-end writes