Developers Guide

10 Understanding RAID with Dell SC Series Storage | 3104-CD-DS
2 RAID rebuild, availability, and efficiency comparison
In general terms, rebuild rates, availability, and capacity efficiency are dependent on the RAID level.
Figure 2 depicts the rebuild rate based on RAID levels for the different SC Series disk options. The results are
from rebuilds with a light random, 64KB, 70-percent read workload simulation.
Rebuild rates for different RAID levels
Observations from Figure 2 show that SSDs have a great advantage over spinning disks for rebuilds with I/O
activity. Spinning disks clearly show the progression of rebuild efficiency between RAID levels. RAID 10
experiences the greatest rebuild rate while RAID 6 is slightly lower than RAID 5.
2.1 SC Series RAID-level disk failure protection and reliability
In principle, RAID levels 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.
Comparing 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 – mean time before failure (MTBF) and annual
failure rate (AFR)
 RAID level – RAID 10, RAID 10-DM, RAID 5-9, RAID 5-5, RAID 6-10, RAID 6-6
 RAID geometry – the RAID set construction or number of disks in the RAID
 RAID rebuild rate – the amount of time to rebuild a disk.
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.
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
RAID 5-9 RAID 6-10 RAID 10
GB/day
RI SSD SAS 15K NLSAS 7.2K