74 Chapter 4
Configuring and Using the 9900 Subsystem
Subsystem counter reports. The cache statistics reflect the logical caching status of the
volumes. For the 9900, Hitachi Data Systems recommends that you set the nonvolatile
storage (NVS) ON and the DASD fast write (DFW) ON for all logical volumes. This will not
affect the way the 9900 caches data for the logical volumes. The default caching status
for the 9900 is:
CACHE ON for the subsystem
CACHE ON for all logical volumes
CACHE FAST WRITE ON for the subsystem
NVS OFF for the subsystem ← Change NVS to ON for the 9900.
DFW OFF for all volumes ← Change DFW to ON for the 9900.
Note: In normal cache replacement, bypass cache, or inhibit cache loading mode, the 9900
performs a special function to determine whether the data access pattern from the host is
sequential. If the access pattern is sequential, the 9900 transfers contiguous tracks from the
disks to cache ahead of time to improve cache hit rate. Due to this advance track transfer,
the 9900 shows the number of tracks transferred from the disks to the cache slot at
DASD/CACHE of the SEQUENTIAL in TRANSFER OPERATIONS field in the subsystem counters
report, even though the access mode is not sequential.
IDCAMS LISTDATA STATUS. The LISTDATA STATUS command generates status information
for a specific device within the subsystem. The 9900 reports two storage sizes:
Subsystem storage. This field shows capacity in bytes of cache. For a 9900 with more
than one SSID, the cache is shared among the SSIDs instead of being logically divided.
This strategy ensures backup battery power for all cache in the 9900. For the 9900, this
field shows three-fourths (75%) of the total cache size.
Nonvolatile storage. This field shows capacity in bytes of random access cache with a
backup battery power source. For the 9900, this field shows one-fourth (25%) of the total
cache size.
IDCAMS SETCACHE. The 9900 supports the IDCAMS SETCACHE commands, which manage
caching for subsystem storage through the use of one command (except for REINITIALIZE).
The following SETCACHE commands work for the subsystem storage across multiple SSIDs:
SETCACHE SUBSYSTEM ON|OFF
SETCACHE CACHEFASTWRITE ON|OFF
SETCACHE NVS ON|OFF
SETCACHE DESTAGE
Note: The SETCACHE REINITIALIZE command reinitializes only the logical subsystem
specified by the SSID. You must issue the REINITIALIZE command once for each defined SSID.
DEVSERV PATHS. The DEVSERV PATHS command is defined as the number of LVIs that can
be specified by an operator (from 1 through 99). To display an entire 9900 subsystem, enter
the DEVSERV command for several LVIs, as follows:
DEVSERV PATHS,100,64
DEVSERV PATHS,140,64
DEVSERV PATHS,180,64
DEVSERV PATHS,1C0,64