Administrator Guide

Linear disk groups
A linear disk group requires the specification of a set of disks, RAID level, disk group type, and a name. Whenever the system
creates a linear disk group, it also creates an identically named linear pool at the same time. No further disk groups can be added
to a linear pool.
For maximum performance, all of the disks in a linear disk group must share the same classification, which is determined by disk
type, size, and speed. This provides consistent performance for the data being accessed on that disk group. To dissolve a linear
disk group, delete the disk group and the contained volumes are automatically deleted. The disks that compose that linear disk
group are then available to be used for other purposes.
The RAID levels for linear disk groups created through the PowerVault Manager must be fault tolerant. The supported RAID
levels for linear disk groups in the interface are: RAID 1, RAID 5, RAID 6, RAID 10, RAID 50 and ADAPT. RAID 10 and RAID
50 only appear in the interface if the system's disk configuration supports them. If RAID 10 is specified, the disk group has a
minimum of two sub-groups. If RAID 50 is selected, depending on the number of selected disks, varying numbers of sub-groups
can be created. Additionally, you can create fault-tolerant RAID-3 or non-fault-tolerant NRAID or RAID-0 disk groups through
the CLI.
NOTE: Tiering, snapshots, and replications are not available for linear pools.
Read-cache disk groups
A read-cache disk group is a special type of a virtual disk group that is used to cache virtual pages to improve read performance.
Read cache does not add to the overall capacity of the pool to which it has been added. You can add or remove it from the pool
without any adverse effect on the volumes and their data for the pool, other than to impact the read-access performance.
If your system uses SSDs, you can create read-cache disk groups for virtual pools if you do not have any virtual disk groups for
the pool that are comprised of SSDs. Virtual pools cannot contain both read-cache and a Performance tier.
Only a single read-cache disk group may exist within a pool. Increasing the size of read cache within a pool requires the user
to remove the read-cache disk group, and then re-add a larger read-cache disk group. It is possible to have a read-cache disk
group that consists of one or two disks with a non-fault tolerant RAID level. For more information on read cache, see About SSD
read cache.
About RAID levels
The RAID controllers enable you to set up and manage disk groups, the storage for which may be spread across multiple
disks. This is accomplished through firmware resident in the RAID controller. RAID refers to disk groups in which part of the
storage capacity may be used to achieve fault tolerance by storing redundant data. The redundant data enables the system to
reconstruct data if a disk in the disk group fails.
For a description of the ADAPT data protection level, see About ADAPT.
NOTE: Choosing the right RAID level for your application improves performance.
The following tables:
● Provide examples of appropriate RAID levels for different applications.
● Compare the features of different RAID levels.
● Describe the expansion capability for different RAID levels (linear disk groups).
● Suggest the number of disks to select for different RAID levels (virtual disk groups).
● Describe the expansion capability for different RAID levels.
NOTE:
To create an NRAID, RAID-0, or RAID-3 (linear-only) disk group, you must use the CLI add disk-group
command. For more information about this command, see the Dell EMC PowerVault ME4 Series Storage System CLI Guide.
NOTE: You can only create RAID-1, RAID-5, RAID-6, and RAID-10 and ADAPT virtual disk groups.
Table 1. Example applications and RAID levels
Application RAID level
Testing multiple operating systems or software development
(where redundancy is not an issue)
NRAID
16 Getting started