Product manual

CDA 7 5630 Product Manual
3-4 77 A1 69UP Rev00
3.1.1.2 Prefetch Algorithm
The Prefetch algorithm dynamically detects sequential data access patterns to the
disk devices. The directors improve the hit ratio of these accesses by promoting
blocks from the disk devices to cache slots before a read miss can occur.
The Prefetch algorithm can stage between a single block and a full track of data to
a cache slot. The LRU algorithm adds the cache slot to the top of the LRU list. To
make room for these tracks, CDA 7 removes slots at the bottom of the list from
cache.
3.1.2 Caching Techniques
CDA 7 uses the same cache principle as host systems, but with enhanced caching
techniques. The following figure illustrates cache use in CDA 7.
ICDA
CHANNEL
DIRECTOR
CACHE
DIRECTORY
DISK
DIRECTOR
DISKS
Figure 3-3. CDA 7 Cache Use
CDA 7 directors handle either channel or disk operations. Channel directors attach
to the CPU channels as well as to cache. Disk directors attach to cache as well as
the disk drives. The CDA 7 directors perform the following functions:
• The channel director handles I/O requests from the host. It accesses the
directory in cache to determine if the request can be satisfied out of cache. The
directory contains information on each cache slot and the blocks within each
slot. It maintains data in cache based on the data access patterns.
• The disk director manages accesses to the disk drives. It performs a background
operation that "destages" “written-to” blocks to disk.