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Dell PowerVault ME4 Series Storage System Administrator’s Guide

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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 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 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
Video editing and production 3
Network operating system, databases, high availability applications, workgroup servers 5
Very large databases, web server, video on demand 50
Mission-critical environments that demand high availability and use large sequential workloads 6
Environments that need flexible storage and fast rebuilds 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
3 3 Block-level data striping with dedicated parity disk Excellent performance for large, sequential data requests (fast read); protects against single disk failure Not well-suited for transaction-oriented network applications; write performance is lower on short writes (less than 1 stripe)
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

50

(5+0)

6 Stripes data across multiple RAID-5 sub-groups Better random read and write performance and data protection than RAID 5; supports more disks than RAID 5; protects against multiple disk failures Lower storage capacity than RAID 5
ADAPT 12 Distributed erasure coding with dual disk failure protection 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
Table 4. Linear disk group expansion by RAID level
RAID level Expansion capability Maximum disks
NRAID Cannot expand. 1
0, 3, 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
50 You can add one sub-group at a time. The added sub-group must contain the same number of disks as each of the existing sub-groups. 32
ADAPT You can add up to 68 disks at a time. 128

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