Administrator Guide

When a read-cache group consists of one SSD, it automatically uses NRAID. When a read-cache group consists of two SSDs, it
automatically uses RAID 0.
For more information on SSDs, see About SSDs on page 19.
About spares
Spare disks are unused disks in your system that you designate to automatically replace a failed disk, restoring fault tolerance to
disk groups in the system. Types of spares include:
● Dedicated spare. Reserved for use by a specific linear disk group to replace a failed disk. Most secure way to provide spares
for disk groups, but expensive to reserve a spare for each disk group.
● Global spare. Reserved for use by any fault-tolerant disk group to replace a failed disk.
● Dynamic spare. Available compatible disk that is automatically assigned to replace a failed disk in a fault-tolerant disk group.
NOTE: You cannot designate spares for ADAPT disk groups. For information on how ADAPT disk groups manage sparing,
see About RAID levels.
A controller automatically reconstructs a fault-tolerant disk group (RAID 1, 3, 5, 6, 10, 50) when one or more of its disks fails
and a compatible spare disk is available. A disk is compatible if it has enough capacity to replace the failed disk and is the same
speed and type (enterprise SAS, for example). It is not advisable to mix 10k and 15k disks in a single disk group. If the disks in
the system are FDE-capable and the system is secure, spares must also be FDE-capable.
When a disk fails, the system looks for a dedicated spare first. If it does not find a dedicated spare, it looks for a global spare. If
it does not find a compatible global spare and the dynamic spares option is enabled, it takes any available compatible disk. If no
compatible disk is available, reconstruction cannot start.
NOTE:
A best practice is to designate spares for use if disks fail. Dedicating spares to disk groups is the most secure
method, but it is also expensive to reserve spares for each disk group. Alternatively, you can enable dynamic spares or
assign global spares.
About pools
A pool is an aggregation of one or more disk groups that serves as a container for volumes. Virtual and linear storage systems
both use pools. A disk group is a group of disks of the same type, using a specific RAID level that is incorporated as a
component of a pool, that stores volume data. For virtual pools, when volumes are added to a pool the data is distributed across
the pool's disk groups. For linear pools, which can only have one disk group per pool, volumes are also added to the pool, which
contains the volume data.
In both virtual and linear storage, if the owning controller fails, the partner controller assumes temporary ownership of the
pool and resources owned by the failed controller. If a fault-tolerant cabling configuration, with appropriate mapping, is used to
connect the controllers to hosts, LUNs for both controllers are accessible through the partner controller so I/O to volumes can
continue without interruption.
You can provision disks into disk groups. For information about how provisioning disks works, see Adding a disk group.
Virtual pools and disk groups
The volumes within a virtual pool are allocated virtually and separated into fixed size pages, with each page allocated randomly
from somewhere in the pool. The volumes within a virtual pool are also thin provisioned, which means that the volumes initially
exist as an entity, but physical storage is not allocated to them. The thin-provisioned volumes are allocated on-demand as data
is written to a page.
If you would like to create a virtual pool that is larger than 512 TiB on each controller, you can enable the large pools feature
by using the large-pools parameter of the set advanced-settings CLI command. When the large pools feature is
disabled, which is the default, the maximum size for a virtual pool is 512 TiB, and the maximum number of volumes per snapshot
tree is 255 (base volume plus 254 snapshots). Enabling the large pools feature will increase the maximum size for a virtual pool
to 1024 TiB (1 PiB) and decrease the maximum number of volumes per snapshot tree to nine, which is the base volume plus
eight snapshots. The maximum number of volumes per snapshot decreases to fewer than nine if more than three replication
sets are defined for volumes in the snapshot tree. For more information about the large-pools parameter of the set
advanced-settings CLI command, see the Dell EMC PowerVault ME4 Series Storage System CLI Guide.
Getting started
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