IBM DS8000 manual Processor complex, Randomseq

RANDOMSEQ

MRU MRU

LRU

RANDOM bottom

LRU

Desired size

SEQ bottom

Figure 2-4 Cache lists of the SARC algorithm for random and sequential data

To follow workload changes, the algorithm trades cache space between the RANDOM and SEQ lists dynamically and adaptively. This makes SARC scan-resistant, so that one-time sequential requests do not pollute the whole cache. SARC maintains a desired size parameter for the sequential list. The desired size is continually adapted in response to the workload. Specifically, if the bottom portion of the SEQ list is found to be more valuable than the bottom portion of the RANDOM list, then the desired size is increased; otherwise, the desired size is decreased. The constant adaptation strives to make optimal use of limited cache space and delivers greater throughput and faster response times for a given cache size.

Additionally, the algorithm modifies dynamically not only the sizes of the two lists, but also the rate at which the sizes are adapted. In a steady state, pages are evicted from the cache at the rate of cache misses. A larger (respectively, a smaller) rate of misses effects a faster (respectively, a slower) rate of adaptation.

Other implementation details take into account the relation of read and write (NVS) cache, efficient de-staging, and the cooperation with Copy Services. In this manner, the DS6000 and DS8000 cache management goes far beyond the usual variants of the LRU/LFU (Least Recently Used / Least Frequently Used) approaches.

2.3 Processor complex

The DS8000 base frame contains two processor complexes. The Model 921 has 2-way processors while the Model 922 and Model 9A2 have 4-way processors. (2-way means that each processor complex has 2 CPUs, while 4-way means that each processor complex has 4 CPUs.)

The DS8000 features IBM POWER5 server technology. Depending on workload, the maximum host I/O operations per second of the DS8100 Model 921 is up to three times the maximum operations per second of the ESS Model 800. The maximum host I/O operations per second of the DS8300 Model 922 or 9A2 is up to six times the maximum of the ESS Model 800.

26DS8000 Series: Concepts and Architecture