Developers Guide

17 Understanding RAID with Dell SC Series Storage | 3104-CD-DS
When the SC array or disk folder consists only of large disks such as 2 TB HDDs or 4 TB SSDs or larger, the
tier is dual redundant. For these situations, RAID 10-DM is used for writes, while RAID 6 is updated by Data
Progression. Consider the performance implications for large Tier 1 disk solutions.
Application I/O writes only to RAID 10
Data Progression, the automated data movement by the SC Series array, frees up as much RAID 10 space
as possible. On-demand data progression moves data in Tier 1 from RAID 10 to RAID 5.
4.3 Highest performing RAID write advantage
The SC Series architecture is designed to write to the highest performing RAID level. This section describes
why RAID 10 is considered the highest performing RAID.
RAID technology applies the basic idea that multiple copies of data protect the larger set of data in the event
of a disk failure. Different RAID configurations require additional I/O during updates to ensure this availability.
For instance, RAID 5 has a single parity disk (or area on a disk) that is used to rebuild the failed disk. RAID 10
simply has a duplicate or mirror disk that is updated with the primary disk. In the case of RAID 5, when a write
occurs, this parity needs to be modified by reading and recalculating and then writing the data back to the
disks. From the theoretical standpoint, RAID 5 requires four I/Os; this is sometimes known as the RAID write
penalty.
The RAID 5 write penalty consists of:
 Read from data
 Read from parity
 Write new parity
 Write new data
RAID 10, on the other hand, only writes the changed data to the primary and mirror disks, and therefore has
about a 50-percent less burden on the disks during writes. The low overhead of operations during writes
comes at the cost of usable capacity (50-percent efficient), however an SC Series array moves data to more
efficient RAID levels or tiers according to their access patterns.