Developers Guide
16 Understanding RAID with Dell SC Series Storage | 3104-CD-DS
For illustrative purposes, Figure 5 shows a possible RAID distribution in a twelve-disk enclosure with one
global hot spare. Tier 1, which includes the Fast Track feature, has both RAID 10 and RAID 5-5 (RAID 5-9 is
also an option).
SC Series example of RAID 5-5 and RAID 10 in Tier 1 over 12 disks with one spare.
The example in Figure 5 shows RAID distributed across all of the disks, and parity is staggered as much as
possible while hot spots are minimized. Reading the graphic from left to right, RAID 5-5 allocates a parity
every fifth disk, starting in the outer tracks or Fast Track regions, then wraps to the standard tracks. RAID 10
follows the same pattern. Notice that in the Fast track region, one segment in D11 is unallocated since RAID
10 needs a two disk relationship. The three unallocated regions in the standard sections of D9-D11 represent
where the next RAID 10 or RAID 5-5 would start if the system needed that area.
4.2 Application data writes
By default, the SC Series array writes all application data to the highest performing RAID level in Tier 1, which
is typically RAID 10, then later moves the data according to the user-defined schedules. Writing to RAID 10 is
an architectural advantage since RAID 10 is the highest performing RAID level for random write profiles.
Since random writes typically have the most impact on any storage environment, RAID 10 minimizes the
effect on performance.
Figure 6 represents access from the application to the various RAID levels in the various tiers. The application
may read from any tier and any RAID level, but only writes to Tier 1 and RAID 10 as long as space is
available.
The SC Series array allows application writes to available space in other tiers and RAID levels if RAID 10 on
Tier 1 is full. If RAID 10 on Tier 1 is full, the following precedence will occur for application writes to the
SC Series array.
If RAID 10 is full, writes will occur on RAID 5.
If Tier 1 disks are full, writes will occur on the next tier.
If Tier 3 RAID 10-DM is full, writes will occur on Tier 3 RAID 6.
Proper design of Tier 1 is essential to maximize the utilization and performance of the SC Series array.