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Dell PowerVault ME5 Series Administrator's Guide

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RAID levels

The 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 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 ADAPT.

NOTE:Choosing the right RAID level for your application improves performance.

In the PowerVault Manager you can create ADAPT, RAID-1, RAID-5, RAID-6, and RAID-10 disk groups. To create an NRAID or RAID-0 (linear only) disk group, you must use the add disk-group CLI command as described in the CLI Reference Guide.

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).
Table 1. Example applications and RAID levelsExample applications and RAID levels
Application RAID level
Testing multiple operating systems or software development (where redundancy is not an issue) NRAID
Fast temporary storage or scratch disks for graphics, page layout, and image rendering 0
Workgroup servers 1 or 10
Network operating system, databases, high availability applications, workgroup servers 5
Mission-critical environments that demand high availability and use large sequential workloads 6
Provides flexible storage and fast rebuilds. Well-suited for most workloads other than those using very few disks, or requiring a high number of writes ADAPT
Table 2. RAID level comparison
RAID level Min. disks Description Strengths Weaknesses
NRAID 1 Non-RAID, nonstriped mapping to a single disk Ability to use a single disk to store additional data Not protected, lower performance (not striped)
0 2 Data striping without redundancy Highest performance No data protection: if one disk fails all data is lost
1 2 Disk mirroring Very high performance and data protection; minimal penalty on write performance; protects against single disk failure High redundancy cost overhead: because all data is duplicated, twice the storage capacity is required
5 3 Block-level data striping with distributed parity Best cost/performance for transaction-oriented networks; very high performance and data protection; supports multiple simultaneous reads and writes; can also be optimized for large, sequential requests; protects against single disk failure Write performance is slower than RAID 0 or RAID 1
6 4 Block-level data striping with double distributed parity Best suited for large sequential workloads; non-sequential read and sequential read/write performance is comparable to RAID 5; protects against dual disk failure Higher redundancy cost than RAID 5 because the parity overhead is twice that of RAID 5; not well-suited for transaction-oriented network applications; non-sequential write performance is slower than RAID 5
10

(1+0)

4 Stripes data across multiple RAID-1 subgroups Highest performance and data protection (protects against multiple disk failures) High redundancy cost overhead: because all data is duplicated, twice the storage capacity is required; requires minimum of four disks
ADAPT 12 Distributed erasure coding with dual disk failure protection supports 16+2 or8+2 stripe width Very fast rebuilds, no spare disks (built-in spare capacity), large storage pools, simplified initial deployment and expansion Requires minimum of 12 disks
Table 3. Number of disks per RAID level to optimize virtual disk group performance
RAID level Number of disks (data and parity)
1 2 total (no parity)
5 3 total (2 data disks, 1 parity disk); 5 total (4 data disks, 1 parity disk); 9 total (8 data disks, 1 parity disk)
6 4 total (2 data disks, 2 parity disks); 6 total (4 data disks, 2 parity disks); 10 total (8 data disks, 2 parity disks)
10 4–16 total
ADAPT 12–128 total
NOTE:RAID 5 and RAID 6 disk groups, which have parity disks, should be created using the "power of 2" method to align properly with virtual pages. Failure to follow this method can result in significant degradation of sequential write performance.

RAID 5 disk groups should be created using 3, 5, or 9 disks. RAID 6 disk groups should be created using 4, 6, or 10 disks.

Table 4. Linear disk group expansion by RAID level
RAID level Expansion capability Maximum disks
NRAID Cannot expand. 1
0, 5, 6 You can add from 1 to 4 disks at a time. 16
1 Cannot expand. 2
10 You can add 2 or 4 disks at a time. 16
ADAPT You can add up to 68 disks at a time. 128

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