• Introduction to pools

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    Introduction to pools

    About pools

    A pool is a set of drives that provide specific storage characteristics for the resources that use them. For example, the pool configuration defines the types and capacities of the drives in the pool. For physical deployments, the pool configuration also defines the RAID configurations (RAID types and stripe widths) for these drives.

    You choose which pool to use when you create a new storage resource.

    Before you create storage resources, you must configure at least one pool. You cannot shrink a pool or change its storage characteristics without deleting the storage resources configured in the pool and the pool itself. However, you can add drives to expand the pool.

    Pools generally provide optimized storage for a particular set of applications or conditions. When you create a storage resource for hosts to use, you must choose a pool with which to associate the storage resource. The storage that the storage resource uses is drawn from the specified pool.

    If there are multiple drive types on the system, you can define multiple tiers for the pool. In physical deployments, each tier can be associated with a different RAID type.

    Storage tiers

    The storage tiers available for both physical and virtual deployments are described in the table below.

    • For physical deployments, the storage tier is associated with the physical drive type.
    • For virtual deployments, the storage tier is associated with the virtual disk's underlying characteristics and must be manually assigned.
    • For both types of deployments, if FAST VP is installed on the system, you can create tiered pools to optimize drive utilization. A tiered pool consists of multiple drive types, such as SAS Flash 2 drives and SAS drives.
    Table 1. Storage tier descriptions
    Storage tier
    Disk types
    Description
    Default RAID configuration (physical deployments only)
    Extreme Performance tier
    Solid state extreme performance drives. The following types are supported:
    • SAS Flash 2
    • SAS Flash 3
    • SAS Flash 4
    Provides very fast access times for resources subject to variable workloads. For example, databases can achieve their best performance when using SAS Flash drives. SAS Flash drives are more expensive than SAS drives per GB of storage.

    SAS Flash 2, SAS Flash 3, and SAS Flash 4 drives can be used together in the Extreme Performance tier. Only SAS Flash 2 drives can be used in the FAST Cache, and only SAS Flash 2 and SAS Flash 3 drives can be used in FAST VP.

    RAID 5 (8 + 1).
    Performance tier
    SAS - Rotating performance drive
    Provides high, all-around performance with consistent response times, high throughput, and good bandwidth at a mid-level price point. Performance tier storage is appropriate for database resources accessed centrally through a network.
    RAID 5 (4 + 1).
    Capacity tier
    NL-SAS - Rotating capacity drive
    Provides the highest storage capacity with generally lower performance. Capacity storage is appropriate for storing large amounts of data that is primarily static (such as video files, audio files, and images) for users and applications that do not have strict performance requirements.
    For data that changes or is accessed frequently, capacity tier storage has significantly lower performance.
    RAID 6 (6 + 2).

    Pool best practices

    Using fewer pools reduces complexity and increases flexibility. However, it may be appropriate to configure multiple pools, in order 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 multi-tenancy.
    • Create smaller failure domains.

    Pools must maintain free capacity in order to operate properly. By default, the storage system raises an alert if a pool has less than 30% free capacity, and will begin to automatically invalidate snapshots and replication sessions if a pool has less than 5% free capacity. It is recommended that a pool always have at least 10% free capacity.

    All-Flash pools

    All-Flash pools provide the highest level of performance in Unity. Use an all-Flash pool for applications that require the highest storage performance at the lowest response time.

    • FAST Cache and FAST VP are not applicable to all-Flash pools.
    • Compression is only supported in all-Flash pools.
    • Snapshots and replication operate most efficiently in all-Flash pools.

    It is recommended that you use a single drive size and a single RAID width within an all-Flash pool.

    Hybrid pools

    Hybrid pools typically provide greater capacity at a lower cost than all-Flash pools, but also typically have 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.

    It is recommended that you provision a Flash tier in hybrid pools. The Flash tier helps enable pool performance efficiencies, and improves response times when using snapshots or replication, or both. The minimum recommended Flash capacity is at least 5% of the pool capacity.

    You can improve the performance of a hybrid pool by increasing the amount of capacity in the Flash tier, so that more of the active dataset resides on and is serviced by the Flash drives.

    Hybrid pools can have up to three tiers (Extreme Performance, Performance, and Capacity). It is recommended that you use a single drive speed, size, and RAID width within a tier of a hybrid pool.

    Spare drive policy (physical deployments only)

    The storage system uses permanent hot sparing to replace a drive that has failed or faulted. Any unused drive in the system with the appropriate drive technology and size can be used to replace a failed or faulted drive in a pool. Most of the drive configurations require the use of hot sparing, except for the following:

    • If the system has only 8 drives in total, and they are of the same type, you can configure RAID 6 (6+2) with no hot spare.
    • If the system has only 12 drives in total, and they are of the same type, you can configure RAID 6 (10+2) with no hot spare.
    When you create or expand pools for a drive configuration, the system prevents you from configuring all available drives, so as to leave some drives as spares.

    The spare drive policy follows these rules to determine how many drives are left as spares:

    • In general, the system reserves one spare drive for every group of 1-31 drives that have the same type, capacity, and rotational speed (or Flash type). For example, if there are 40 300-GB, 15K-RPM SAS drives in the system, the system reserves two of those drives as spares.
    • The system does not reserve a spare drive for:
      • The FAST Cache.
      • A system drive, unless it has user data on it.
    • Any unused non-system drive can become a spare drive.
    • A system drive that does not contain user data can be a spare drive for a failed system drive that has user data.
    • A spare drive can be used to replace a failed or faulted drive in the FAST Cache.
    • When a spare drive swaps into a pool, it becomes a permanent member of that pool and cannot be used in another pool.
    • When a broken drive is fixed or replaced, it can be a candidate for a spare drive or used in another pool.

    Refer to the compatibility and interoperability documentation on the support website for a listing of basic platform and component support for the storage system, including capacity limits.

    About RAID groups (physical deployments only)

    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.

    A RAID group is a set of drives with the same capacity and redundancy on which you create one or more storage resources. For example, when you create a storage resource in a RAID 5 group, data is distributed equally across the drives in the RAID group. The following illustration shows a five-drive RAID 5 group with three LUNs:

    Figure 1. Five-drive RAID 5 group with three LUNs
    An illustration that shows a five-drive RAID 5 group with three LUNs.

    RAID groups usually have 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.

    You select drive types and RAID configurations (RAID types and stripe widths) for a pool. The system then creates one or more RAID groups for the pool, based on the specified configuration. For example, assume there is one storage tier in the pool. Using the five-drive RAID 5 example shown above, the system would create the pool with a single RAID 5 (4+1) RAID group. If you want to create the pool with more than five drives, you must do so in increments of five. Alternatively, you can select the Maximum Capacity option, which optimizes based on number of drives selected and may create multiple RAID groups of the same RAID type but different stripe widths.

    If there are unused drives of different types, you can configure multiple storage tiers for the pool and can specify a different RAID configuration for each tier.

    Once a pool is configured, you cannot change the RAID type of a tier. However, you can add a new tier with a different RAID type.

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

    RAID configurations (physical deployments only)

    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. A RAID 5 (4+1), 5 drive configuration has less risk of multiple drive faults than a RAID 5 (12+1), 13 drive configuration.

    For best performance from the least number of drives, match the appropriate RAID level with the expected workload. The following table describes the supported RAID types for the intended storage usage:

    Table 2. Supported RAID levels
    RAID level
    Description
    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 1/0 requires a minimum of two physical drives to implement, where two drives are mirrored together to provide fault tolerance. A RAID 1/0 configuration can continue to operate as long as 1/2 of each mirrored drive pair is healthy.

    For example, if you have a RAID (2+2) configuration, you can lose two drives, as long as they are not the source and mirror of the same mirrored pair. If you lose both the source and mirror of the same mirrored pair, you must immediately replace the drives and rebuild the array.

    • RAID 1/0 (1+1): A minimum of two drives can be allocated at a time to a pool, with one used strictly for mirroring. One drive out of every two is an exact duplicate of the other, and the usable drive capacity for every two-drive group is approximately one drive (50%). This RAID configuration is equivalent to RAID 1.
    • RAID 1/0 (2+2): A minimum of four drives can be allocated at a time to a pool, with two used strictly for mirroring. Two drives out of every four are exact duplicates of the other, and the drive usable drive capacity for every four-drive group is approximately two drives (50%). In addition, this configuration is striped to improve I/O performance.
    • RAID 1/0 (3+3): A minimum of six drives can be allocated at a time to a pool, with three used strictly for mirroring. Three drives out of every six are exact duplicates of the other, and the drive usable drive capacity for every six-drive group is approximately three drives (50%). In addition, this configuration is striped to improve I/O performance.
    • RAID 1/0 (4+4): A minimum of eight drives can be allocated at a time to a pool, with four used strictly for mirroring. Four drives out of every eight are exact duplicates of the other, and the drive usable drive capacity for every eight-drive group is approximately four drives (50%). In addition, this configuration is striped to improve I/O performance.
    A failure of a mirrored pair in a RAID 1/0 drive group will render any storage in the RAID group unavailable until the failed drives are replaced and the data is restored. This may cause data loss since the last backup of the pool.
    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 5 stripes data at a block level across several drives and distributes parity among the drives. No single drive is devoted to parity. Because parity data is distributed on each drive, read performance can be lower than with other RAID types.

    RAID 5 requires all drives but one to be present to operate. If a drive fails, it will reduce storage performance and should be replaced immediately. Data loss will not occur as a result of a single drive failure.

    • RAID 5 (4+1): A minimum of five drives can be allocated at a time to each pool. The usable capacity for every five-drive group is approximately four drives (80%).
    • RAID 5 (8+1): A minimum of nine drives can be allocated at a time to each pool. The usable capacity for every nine-drive group is approximately eight drives (89%).
    • RAID 5 (12+1): (Not for general use) A minimum of 13 drives can be allocated at a time to each pool. The usable capacity for every 13-drive group is approximately 12 drives (92%). This configuration should only be used when lower data protection characteristics are acceptable.
    A failure of two drives in a RAID 5 drive group will render any storage in the RAID group unavailable until the failed drives are replaced and the data is restored. This may cause data loss since the last backup of the pool.
    RAID 6

    Best suited for read-biased workloads, such as archiving and backup to drive.

    RAID 6 is similar to RAID 5, but includes a double parity scheme that is distributed across different drives and thus offers extremely high fault- and drive-failure tolerance. RAID 6 also provides block-level striping with parity data distributed across all drives.

    The pool will continue to operate even when up to two drives fail. Double parity provides time to rebuild the RAID group, even if another drive fails before the rebuild is complete.

    • RAID 6 (6+2): A minimum of eight drives can be allocated at a time to each pool. The usable capacity for every eight-drive group is approximately six drives (75%).
    • RAID 6 (8+2): A minimum of 10 drives can be allocated at a time to each pool. The usable capacity for every ten-drive group is approximately eight drives (80%).
    • RAID 6 (10+2): A minimum of 12 drives can be allocated at a time to each pool. The usable capacity for every 12-drive group is approximately 10 drives (83%).
    • RAID 6 (14+2): A minimum of 16 drives can be allocated at a time to each pool. The usable capacity for every sixteen-drive group is approximately fourteen drives (88%).
    A failure of three drives in a RAID 6 drive group will cause data loss and render any storage in the RAID group unavailable until the failed drives are replaced and the data is restored. This may cause data loss since the last backup of the pool.
    Mixed RAID configurations

    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.

    Disk IOPS by RAID type (physical deployments only)

    Front-end application workloads translate into different back-end disk 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 3. 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