IBM 5655-DB2, 5695-DF1 manual Cache Subsystem Status, Cache Subsystem Overview

There are three Cache Subsystem Activity reports:

•Cache Subsystem Status

This report gives the amount of cache storage and nonvolatile storage (NVS) installed, as well as the current status of the cache.

•Cache Subsystem Overview

This report gives the number of I/O requests sent to the control unit and their resolution in the cache (hits).

•Cache Subsystem Device Overview

This report gives, for all online volumes attached to the subsystem, the specific utilization of the cache. It also consolidates this information at the LCU level. This information is often correlated with the LCU view of the DEVICE report.

These statistics reports are generated by each disk storage server from IDCAMS LISTDATA command requests to each storage server LCU. Therefore, subsystem identification is by SSID or a control unit identifier (CU-ID). CU-ID is the lowest online device number attached to the LCU. This pinpointing is used to establish correlations between cache and device activity reports. Moreover, all reported values are consolidated data from all LPARs sharing the LCU. Figure 48 on page 127 shows cache subsystem status and overview reports, and Figure 49 on page 127 shows the cache subsystem device overview report.

Cache Subsystem Status

CACHING must be ACTIVE.

Cache Subsystem Overview

This report consists of consolidated sets of information:

•READ I/O REQUESTS, WRITE I/O REQUESTS, and CACHE MISSES have roughly the same structure. SEQUENTIAL lines report a workload measurement of asynchronous activities. NORMAL lines report synchronous activities.

•Under MISC, the DFW BYPASS field reports NVS overuse. ASYNC (TRKS) displays the data flow between cache and physical disks. A high value of ASYNC I/Os with a BYPASS=0 is an indicator of a heavy workload, but the NVS buffer is adequate.

•Under NON-CACHE I/O, ICL is at zero when DCME is not used.

•The CKD STATISTICS column reports the existence of old channel programs, which can cause performance degradation if they are still used. Some system-related tools can still use those channel programs.

•Under CACHE MISSES there are four sets of data:

•NORMAL and SEQUENTIAL lines show respectively synchronous and asynchronous I/O misses.

•The TRACKS and RATE columns display staging activity from physical disks to cache. In particular, the sequential prefetch activity is accounted by number of tracks read and by rate of the read at the end of the SEQUENTIAL line.

•CFW DATA is positive when DFSORT uses cache sortwork files.

•TOTAL covers read and write columns only.

126Storage Management with DB2 for OS/390