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6 Dell PS Series Storage: Choosing a Member RAID Policy | TR1020 | v 4.7
2 RAID policy availability and performance comparisons
Although all RAID levels provide good performance and data protection, there are some differences. When
choosing a member RAID policy, the performance and availability needs of the workload should be identified
to select a RAID policy that meets those needs. If the workload has mixed requirements in terms of
performance and availability, consider mixing RAID types in a multi-member group.
There are many factors to consider when choosing a RAID level for a PS Series member, as shown in Table
3. The first column lists workload requirements, with the other columns listing the relative position of each
requirement for each RAID policy.
RAID policy comparison.
RAID
Reads
(Random/Sequential
Writes
(Random/Sequential
Relative
Cost
Relative
Protection
Rebuild
Performance
10
Excellent/Excellent
Excellent/Good
Highest
Excellent
Best
1
50
Excellent/Good
Good/Good
Moderate
Adequate
Good
6
Excellent/Good
OK/Good
Moderate
Best
OK
5
Excellent/Good
OK/Good
Lowest
Poor
OK
1
RAID 10 is not recommended for any SSD or HDD larger than 6 TB.
Note: RAID performance and availability is directly affected during rebuild and reconstruction. RAID
reconstruction times increase substantially as physical disk sizes increase. The time for reconstruction
increases the potential for a second disk failure which exposes the vulnerability of the data on the array.
2.1 RAID policy disk failure protection and reliability
In principle, RAID policies are primarily designed to protect against data loss with various levels of
performance and capacity utilization. That data protection capability is provided by the ability of a RAID set to
rebuild a failed disk while still servicing I/O requests.
A comparison of the resilience of different RAID policies to protect against data loss in the event of a disk
failure relies on a statistical analysis involving the following factors:
 Disk protocol, size, and RPM – physical characteristics of the disk
 Disk failure rates reflecting mechanical reliability – MTBF
 Controller RAID policy – member RAID setting
 RAID geometry – how RAID is constructed on the member
While there are modest variations in relative capacity utilization and performance, the levels of data protection
provided by RAID policies vary dramatically, even when considering the same type of disk. The graphic below
depicts the relative reliability of RAID polices for the different disk options for PS Series arrays.