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356 DS6000 Series: Concepts and Architecture
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
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 DS6000 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.
Connecting via SAN switches
When connecting DS6000 systems to iSeries via switches, you should plan that I/O traffic
from multiple iSeries adapters can go through one port on a DS6000 and zone the switches
accordingly. DS6000 host adapters can be shared between iSeries and other platforms.
RAID rank type Disk ops/sec Host I/O/sec
(70% read)
Host I/O/sec
(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