IBM DS8000 manual Recommended number of ranks, Sharing ranks between iSeries and other servers

Recommended number of ranks

As a general guideline, you may consider 1500 disk operations/sec for an average RAID rank.

When considering the number of ranks, take into account the maximum disk operations per second per rank as shown in Table B-3.These are measured at 100% DDM Utilization with no cache benefit and with the average I/O being 4KB. Larger transfer sizes will reduce the number of operations per second.

Based on these values you can calculate how many host I/O per second each rank can handle at the recommended utilization of 40%. This is shown for workload read-write ratios of 70% read and 50% read in Table B-3.

Table B-3 Disk operations per second per RAID rank

RAID rank type			Disk ops/sec	Host I/O /sec	Host I/O /sec
				(70% read)	(50% read)
RAID-5	15K RPM (7 + P)		1700	358	272
RAID-5	10K RPM (7 + P)		1100	232	176
RAID-5	15K RPM (6 + P + S)		1458	313	238
RAID-5	10K RPM (6 + P + S)		943	199	151
RAID-10		15K RPM (3 + 3 + 2S)	1275	392	340
RAID-10		10K RPM (3 + 3 + 2S)	825	254	220
RAID-10		15K RPM (4 + 4)	1700	523	453
RAID-10		15K RPM (4 + 4	1100	338	293

As can be seen in Table B-3,RAID-10 can support higher host I/O rates than RAID-5. However, you must balance this against the reduced effective capacity of a RAID-10 rank when compared to RAID-5.

Sharing ranks between iSeries and other servers

As a general guideline consider using separate extent pools for iSeries workload and other workloads. This will isolate the I/O for each server.

However, you may consider sharing ranks when the other servers’ workloads have a sustained low disk I/O rate compared to the iSeries I/O rate. Generally, iSeries has a relatively high I/O rate while that of other servers may be lower – often below one I/O per GB per second.

As an example, a Windows file server with a large data capacity may normally have a low I/O rate with less peaks and could be shared with iSeries ranks. However, SQL, DB or other application servers may show higher rates with peaks, and we recommend using separate ranks for these servers.

Unlike its predecessor the ESS, capacity used for logical units on the DS8000 can be reused without reformatting the entire array. Now, the decision to mix platforms on an array is only one of performance, since the disruption previously experienced on ESS to reformat the array no longer exists.

Appendix B. Using DS8000 with iSeries 399