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26 DS8000 Series: Concepts and Architecture
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.
RANDOM
LRU
MRU
RANDOM bottom
SEQ
LRU
MRU
SEQ bottom
Desired size