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Dell Unity supports 2 different types of storage pools, traditional pools and dynamic pools. The following recommendations are applicable to both types of pool.
In general, it is recommended to use fewer storage pools within Dell Unity, as this reduces complexity and increases flexibility. However, it may be appropriate to configure multiple storage pools, in order to:
Storage pool capacity is used for multiple purposes:
Storage pools must maintain free capacity in order to operate properly. By default, Dell Unity will raise an alert if a storage pool has less than 30% free capacity, and will begin to automatically invalidate Snapshots and Replication sessions if the storage pool has less than 5% free capacity. Dell Technologies recommends that a storage pool always have at least 10% free capacity.
Dell Unity applies RAID protection to the storage pool in order to protect user data against drive failures. Choose the RAID type that best suits your needs for performance, protection, and cost:
Traditional Storage Pools apply RAID protection to discrete groups of drives within the storage pool. Traditional pools are available on Dell Unity hybrid systems, and all-Flash systems.
For traditional pools, Dell Technologies generally recommends RAID-5 for drives in Extreme Performance and Performance tiers, and RAID-6 for drives in the Capacity tier.
Assuming that roughly the same number of drives will be configured in a traditional pool, Dell Technologies generally recommends smaller RAID widths as providing the best performance and availability, at the cost of slightly less usable capacity.
EXAMPLE: When configuring a traditional pool tier with RAID-6, use 4+2 or 6+2 as opposed to 10+2 or 14+2.
When choosing RAID-1/0, 1+1 can provide better performance with the same availability and usable capacity as larger RAID widths (assuming that the same total number of drives are used), and also provides more flexibility.
All-Flash pools provide the highest level of performance in Dell Unity. Use an all-Flash pool when the application requires the highest storage performance at the lowest response time.
Snapshots and Replication operate most efficiently in all-Flash pools.
FAST Cache and FAST VP are not applicable to all-Flash pools.
Dell Technologies recommends using only a single drive size and a single RAID width within an all-Flash pool.
EXAMPLE: For an all-Flash pool, use only 1.6TB SAS Flash 3 drives, and configure them all with RAID-5 8+1.
Hybrid pools can contain HDDs (SAS and NL-SAS drives) and Flash and can contain more than one type of drive technology in different tiers. 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. For best performance, it is recommended to have neighboring tiers when creating Hybrid Pools. For example, a Hybrid pools should be configured as Flash and SAS drive or Flash, SAS, and NL-SAS drives. Use hybrid pools for applications that do not require consistently low response times, or that have large amounts of mostly inactive data.
Dell Technologies recommends provisioning a Flash tier in hybrid pools. The Flash tier helps enable pool performance efficiencies, and improves response times when using Snapshots and/or Replication. The minimum recommended Flash capacity is at least 10% of the pool capacity for data reduction purposes. See Data Reduction section.
Performance of a hybrid pool can be improved 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. See the FAST VP section.
Hybrid pools can have up to 3 tiers (Extreme Performance, Performance, and Capacity). Dell Technologies recommends using only a single drive speed, size, and RAID width within each tier of a hybrid pool.
EXAMPLE:
The first 4 drives of every Dell Unity system are called the system drives (DPE Disk 0 through DPE Disk 3). Dell Unity uses capacity from these 4 drives to store copies of configuration information and other critical system data. Therefore, the available capacity from each of these drives is about 107GB less than from other drives. System drives can be added to storage pools like any other drive, but offer less usable capacity due to the system partitions. To reduce the capacity difference when adding the system drives to a pool, Dell Technologies recommends using a smaller RAID width for the tier which will contain the system drives.
EXAMPLE: Choose RAID-5 4+1 for a tier containing the system drives, instead of 12+1.
Due to the capacity difference of the system drives, Dell Unity will not add them to storage pools until all other non-spare drives of the same type have been used.
When used in traditional storage pools, the system drives are counted by Dell Unity in determining how many hot spare drives need to be reserved but note that unbound system drives cannot be used as hot spare drives.
The system drives also service a modest IOPS workload related to system management functions. For systems with Flash system drives, this workload is negligible. Dell Unity Hybrid Flash arrays can use HDD (SAS or NL-SAS) as system drives. When used in a storage pool, these HDD system drives may have a lower performance capability than other HDD drives of the same type.
Consider not using the system drives in storage pools for large configurations with high drive counts and many storage objects, if the system drives are HDD.