Home > Storage > Unity XT > Storage Admin > Dell Unity Dynamic Pools > Spare space extents
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 within each tier of the pool. The number of spare space extents reserved within a dynamic pool directly depends on the number of drives within the pool, the types of drives within the pool, the size of the drives within the pool, and the hot spare capacity setting. The hot spare capacity setting is new in Dell UnityOS version 5.1 release.
The hot spare capacity setting allows a user to reserve either 1 drive (default) or 2 drives worth of spare space for every 32 drives within a pool at the time of pool creation. If the pool being created has multiple tiers, the hot spare capacity can be set for each drive tier during pool creation. The hot spare capacity setting can also be set for a new drive tier when expanding a pool. When creating or expanding a pool the minimum number of drives required for each tier depends on the stripe width and the hot spare capacity set for the tier. The amount of spare space reserved always ensures that either one or two drives of the largest drive’s usable capacity within each tier can be replaced with the spare space extents remaining within the pool. Reserving additional spare space does not improve the reliability of the RAID protection for the pool. Spare space extents within a tier are only used to replace extents within the same tier.
Spare space is counted as part of a pool’s overhead, as with RAID overhead, and is not reported to the user. Spare space is also not part of the usable capacity within the pool for user data. If a pool were to become full and a drive failure occurred, there would still be spare-space extents within the pool to complete the rebuild operation. The amount of spare space within the pool is reserved at the time of pool creation and when expanding a pool. It cannot be modified on an existing pool. When a drive count crosses the 32 drive count, additional spare space extents are reserved from the additional capacity being added to the pool based on the hot spare capacity setting. When expanding a pool in Unisphere in this instance, the amount of usable capacity would show less than that total amount of drive capacity being added to the pool.
Figure 5 shows an example of a 6-drive dynamic pool. Each drive extent in this example is marked with a Dx DE y label signifying which drive and which extent it is, such as D1 DE 1 (drive 1, drive extent 1). At time of creation, several extents on each drive are reserved as spare space extents, denoted in the figure as Spare. Spare space extents are also reserved when adding a new tier to a pool. The amount of spare space extents directly depends on the hot spare capacity setting for the tier. The figure shows that spare space extents are evenly spread across the drives within the pool. For demonstration purposes, all spare space extents are shown at the end of usable capacity of each drive. This is not necessarily the way extents are allocated by the dynamic pool algorithms.