92 Storage Management with DB2 for OS/390
9.3.3 Write Record Caching (Quickw rite)
Write record caching, called quickwrite, extends the benefits of DFW to data that
doesnot havea read-before-update access pattern (whic h is currently required to
havea D FW hit) and for data with a poor cache hit rate. Data with a predictable
record format, such as VSAM records,c anbenef it fromthis algori thm.
9.3.4 Seque ntial Caching
Sequential access to data is managed by a sequential caching algorithm that
prestages tracks ahead of the record requested by the application. Once
accessed, the space occupied by those tracks is marked as available for reuse
rather than being subjected to the LRU algorithm (the exception is the 3990
controller). DB2, through the Media Manager, by requesting sequential caching,
optimizes the cache management forbet ter performance.
9.3.5 No Caching—Bypass Cache
Applications that donot benefit from caching can specify the bypass cache option
in the Define Extent command. This setting is accomplished by the access
methods. Most of the storage servers implementbypass cache by caching the
tracksof data anyway, but also managing LRU algorithms fora faster reutilization
of the cache space memory those tracks occupy.
9.3.6 No Caching—Inhibit Cache Load
A variant of bypass cache isthe inhibit cache load (I CL)comm and.This
command specifies that if the data is found in the cache, it can be read from the
cache, butif not, it should not to be staged into the cache. This may be of benefit
when the same data is accessed in severaldifferent modes, for example, read by
a sequential prefetch operation as well as a random read operation.
9.3.7 DB2 Cache Parameters (DSNTIPE)
DB2 interfaces I/O through the VSAM Media Manager and uses the ICL
command to optimizet hesequential processes. The setting is done at DB2
installation time in the DSNTIPE panel. For the best large cache utilization, we
recommend the SEQUENTIAL CACHE parameter being specified to SEQ
(instead of default BYPASS)for DB2 prefetch. The UTILITY CACHE OPTION
parameter should be set to YES (insteadof default NO).
9.3.8 Dynamic Cache Management Enhancement
DynamicCache Management Enhanced (DCME) is an interactive cache resource
management algorithm between System Managed Storage (SMS) and storage
server Licenced Internal Code (LIC). SMSspecifies for each data set the level of
performance required:
• Whether the data set should be cached (must cache)
• Whether the data set should be excluded from caching (never cache)
• Whether caching should be used only if it is suitable to do so (may cache)
The recommendation is to defineall data sets requiring performance as must
cache (which is accomplished when an ACS routine sets them in a Storage Class
defined with a low response time). This means that while all data sets use
caching, the mustcache data sets have an optimized LRU algorithm, which