Administrator Guide

Because data is characterized every five seconds and moved to the appropriate storage device, no fixed rule is used to determine which
SSDs are used. For this reason, using SSDs with the same DWPD values is advised.
About SSD read cache
Unlike tiering, where a single copy of specific blocks of data resides in either spinning disks or SSDs, the Read Flash Cache (RFC) feature
uses one SSD read-cache disk group per pool as a read cache for frequently accessed data only. Each read-cache disk group consists of
one or two SSDs with a maximum usable capacity of 4TB. A separate copy of the data is also kept in spinning disks. Read-cache content is
lost when a controller restart or failover occurs. Taken together, these attributes have several advantages:
• The performance cost of moving data to read-cache is lower than a full migration of data from a lower tier to a higher tier.
• Read-cache does not need to be fault tolerant, potentially lowering system cost.
• Controller read cache is effectively extended by two orders of magnitude, or more.
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 18.
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.
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Getting started