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128 Storage Management with DB2 for OS/390
Cache Subsystem Device Overview
This report lists the devices known by the subsystem at the beginning of the
interval. Each line displays statistics for a specific (functional) volume. The I/O
rate, divided into two groups (CACHE HIT and DASD I/O), shows the different
types of I/O activity in each group. The *ALL line consolidates values at the
subsystem level. The fields to review are, in decreasing order of importance:
• I/O RATE, number of I/O requests per second.
• % READ, percentage of read requests compared to all read plus write
requests. When combined with DEVICE ACTIVITY RATE of LCU level
DEVICE report, this value enables you get to the WRITE ACTIVITY RATE by:
• (100 - %READ) * DEVICE ACTIVITY RATE / 100
• The write activity rate value is an important factor for performance evaluation
in a remote copy environment.
• READ H/R, read hit ratio.
• WRITE H/R, write hit ratio.
• DFW, rate of DFW requests.
• STAGE, rate of any (normal or sequential and read or write) I/O requests with
cache miss.
• ASYNC RATE, number of tracks asynchronously destaged from cache to disk
as a natural consequence of least recently used cache management
algorithms.
• ICL, rate of inhibit cache load requests should be at zero. See 9.3.6, “No
Caching—Inhibit Cache Load” on page 92.
11.2.1.2 Direct Access Device Activity Report
This report can be produced at either the LCU level (standard) or the Storage
Group level for each LPAR.
Both reporting levels should be used. Although the
LCU report is relevant with CACHE reports, Storage Group level reporting
automatically consolidates information in a global view of I/O activity consistent
with the installation organization defined to SMS. With appropriate SMS
definitions, such Storage Group level reporting should map the applications point
of view.
To get standard LCU reporting, specify:
REPORTS ( DEVICE ( DASD ) )
To get Storage Group reporting, specify in the RMF Monitor III postprocessor:
REPORTS ( DEVICE ( SG ( storage-group-name ) ) )
Figure 50 on page 129 shows the Direct Access Device Activity Report at the
Storage Group (SG) level.
The response time of a specific volume in a given LPAR consists of
service time
and volume thread queuing time. Service time splits into:
•
Pending time, which covers the channel subsystem and connections delays to
the LCU due to ESCON Director switching, and, in ESCON multiple image
facility (EMIF), channel sharing between LPARs