Developers Guide

32 Understanding RAID with Dell SC Series Storage | 3104-CD-DS
7 Design considerations
Once the storage requirements have been determined, a solution may be designed with typically two or three
tiers of storage. However, some environments may only use a single tier. Most often in multi-tiered solutions,
the top tier is sized for 100 percent of the I/O and 20 to 30 percent of the capacity. However, specific
customer needs may require deviation from this starting point. These tiers may consist of SSDs, HDDs, or a
combination of the two. For example, in an all-SSD solution, Tier 1 consisting of write-intensive SSDs would
be allocated as RAID 10, while Tier 2 containing read-intensive SSDs would be RAID 5. This solution could
be further expanded with a third tier consisting of 7.2K HDDs for storage of cold data. Another possible
solution consisting of only HDDs might have a 15K Tier 1, a 10K Tier 2, and a 7.2K Tier 3. The potential
combinations are numerous and are best selected by consulting with a Dell Storage specialist or reseller.
7.1 SSD considerations
SSDs have increased in capacity while maintaining excellent performance. Dell has included 3.84 TB SSDs
which may be used as Tier 1 disks. The default for 3.84 TB SSDs is dual redundancy, however that may be
modified to single redundancy based on analysis of performance data or business needs. For disks 4 TBs
and larger, dual redundancy is required, these SSDs would be configured in Tier 1 if no write-intensive SSDs
exist.
7.2 SSDs in Tier 1 dual redundancy
Since dual redundancy affects the write penalty, consider a modest performance impact when designing with
larger SSDs. The SC Series design criteria calls for Tier 1 to handle the IOPS load with additional disks for
capacity. An SC Series array is designed to accommodate all IOPS in RAID 10. In the following definitions,
Total IOPS references the back-end IOPS expected to the disks.
Single redundant sizing using RAID 10: Total IOPS = reads + writes * 2
When Tier 1 is dual redundant, the basic formula changes for the total IOPs supported. Since an additional
mirror is needed for RAID 10-DM the write penalty is three.
Dual redundant sizing using RAID 10-DM: Total IOPs = reads + writes * 3
When designing solutions using large capacity SSDs, consider the total IOPS burden in high write
environments.
When mixing large SSDs (greater than 3.9 TBs) with HDDs, the SSDs are in Tier 1 as RAID 10-DM and RAID
6. The HDDs are in Tier 2, Tier 3, or both; the redundancy level depends on the capacity of the disks.
Design SC Series arrays with the appropriate Dell Storage specialist to arrive at a solution that best fits the
workloads and business requirements.
7.3 Techniques for specific workloads
SC Series storage is designed to handle applications with many different workloads. However, some
workloads may need to deviate from the SC Series defaults and work with the available features of the array.