Cache subsystem status and overview

The first section shows the configuration of the selected storage subsystem. This includes the amount of cache configured (installed) and available for use, and how much storage, if any, is pinned because of a DASD failure. It also includes the amount of non-volatile storage needed for the DASD FAST WRITE function. Finally, the overall subsystem status is shown in terms of whether caching is active, or, for example, CACHE FAST WRITE is activated.

The second section provides details of the cache usage of the subsystem. Here, the analyst can see at a glance all the I/O requests to the subsystem, divided into the categories NORMAL, SEQUENTIAL, and CACHE FAST WRITE. READ and WRITE requests are shown separately as totals, rates, or ratios.

Note: All values shown as RATE are calculated on the basis of seconds that are shown in the CINT value in the report header.

Figure 1. Cache Subsystem Activity Report - Status and Overview

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            z/OS V2R1               SYSTEM ID   SYS1             DATE 09/28/2013            INTERVAL 15.00.000
                                    RPT VERSION V2R1 RMF         TIME 00.30.00

SUBSYSTEM  2105-01     CU-ID  1E91     SSID 3007     CDATE  09/28/2013     CTIME 00.29.55      CINT  15.00
TYPE-MODEL 2105-E20    MANUF  IBM      PLANT  75     SERIAL 000000016374

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                                                       CACHE SUBSYSTEM STATUS
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SUBSYSTEM STORAGE           NON-VOLATILE STORAGE        STATUS
CONFIGURED     3072.0M      CONFIGURED      192.0M      CACHING                  - ACTIVE
AVAILABLE      2692.2M      PINNED            0.0       NON-VOLATILE STORAGE     - ACTIVE
PINNED            0.0                                   CACHE FAST WRITE         - ACTIVE
OFFLINE           0.0                                   IML DEVICE AVAILABLE     - YES

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                                                      CACHE SUBSYSTEM OVERVIEW
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TOTAL I/O   185787   CACHE I/O   185787   CACHE OFFLINE        0
TOTAL H/R    0.893   CACHE H/R    0.893

CACHE I/O    -------------READ I/O REQUESTS-------------    ----------------------WRITE I/O REQUESTS----------------------         %
REQUESTS       COUNT     RATE      HITS     RATE     H/R      COUNT     RATE      FAST     RATE      HITS     RATE     H/R      READ

NORMAL        151012    167.0    132096    146.1   0.875       5339      5.9      5339      5.9       5339     5.9   1.000      96.6
SEQUENTIAL     24588     27.2     23696     26.2   0.964       4848      5.4      4848      5.4       4848     5.4   1.000      83.5
CFW DATA           0      0.0         0      0.0     N/A          0      0.0         0      0.0          0     0.0     N/A       N/A

TOTAL         175600    194.2    155792    172.3   0.887      10187     11.3     10187     11.3      10187    11.3   1.000      94.5

-----------------------CACHE MISSES-----------------------            ------------MISC------------          ------NON-CACHE I/O-----
REQUESTS      READ    RATE   WRITE    RATE  TRACKS    RATE                            COUNT   RATE                      COUNT   RATE
                                                                      DFW BYPASS          0    0.0          ICL             0    0.0
NORMAL       18916    20.9       0     0.0   25086    27.8            CFW BYPASS          0    0.0          BYPASS          0    0.0
SEQUENTIAL     892     1.0       0     0.0   36075    39.9            DFW INHIBIT         0    0.0          TOTAL           0    0.0
CFW DATA         0     0.0       0     0.0                            ASYNC (TRKS)    18546   20.5

TOTAL        19808    RATE    21.9



---CKD STATISTICS---     ---RECORD CACHING---                         ----HOST ADAPTER ACTIVITY---      --------DISK ACTIVITY-------
                                                                                      BYTES  BYTES               RESP   BYTES  BYTES
WRITE              0     READ MISSES        0                                          /REQ   /SEC               TIME    /REQ   /SEC
WRITE HITS         0     WRITE PROM       472                         READ              982   1.1K      READ   16.912   65.9K  12.7M
                                                                      WRITE           15.9K   2.8M      WRITE  12.465   48.3K   4.1M

Table 1. Fields in the Cache Subsystem Activity Report - Header
Field Heading	Meaning
SUBSYSTEM	Storage subsystem type (as configured).
CU-ID	Physical control unit number of the caching subsystem. This is equal the lowest device number, or to the device that has been turned online first, respectively.
SSID	Subsystem identifier: a number assigned during installation of the subsystem that uniquely identifies the storage subsystem.
Note: Device reserve activity can cause a data gatherer interface to wait until a reserve has been released. This in turn can cause the cache interval to be much longer than a regular RMF interval. Therefore, CDATE, CTIME and CINT have been introduced to show the actual point in time to which the cache interval start is related, and the actual cache interval length. All rates shown in the report are based on CINT, not on INTERVAL.
CDATE	Date when the cache interval started.
CTIME	Time when the cache interval started.
CINT	Cache interval time. In interval reports, the format is mm.ss, while in duration reports the format is hh.mm.ss.
TYPE-MODEL	Device type and model.
TYPE MODEL MANUF PLANT SERIAL	The hardware description of the disk system.
VOLSER	Volume serial number of the reported DASD device (only for device-level reporting).
NUM	Device number of the reported DASD device (only for device-level reporting).

Table 2. Fields in the Cache Subsystem Activity Report - Subsystem Status
Field Heading	Meaning
Note: The reported storage capacities for cache and non-volatile storage (NVS) represents only the Cluster Processor Complex in the storage server, that controls the subsystem. Since a typical storage server has two clusters, you must double the reported capacities to get the actual sizes.
SUBSYSTEM STORAGE	Physical capacity of random access cache (in megabytes).
CONFIGURED	Amount of storage that is installed in this storage subsystem.
AVAILABLE	Amount of storage that is available for caching. This is the total cache size minus the amount used by the subsystem for the cache directory, minus the amount pinned and offline storage.
PINNED	Amount of storage that is unavailable because a DASD failure is preventing the subsystem from destaging the data. The data is pinned in cache.
OFFLINE	Amount of storage that is offline because of a host or subsystem error.
NON-VOLATILE STORAGE (NVS)	Physical capacity of random access storage with a backup battery power source (in megabytes).
CONFIGURED	Amount of NVS that is installed in this storage subsystem.
PINNED	Amount of NVS that is unavailable because a DASD failure is preventing the subsystem from destaging the data. The data is pinned in NVS.
STATUS	The caching status of the entire subsystem.
CACHING	Overall caching status of the subsystem. ACTIVE Caching is active (online and usable). DISABLED FOR MAINTENANCE Cache has been disabled for maintenance. PENDING ACTIVE STATE Caching is pending active, that is, cache is in the process of being brought online. INTERNAL ERROR TERMINATION An internal error stopped cache (cache is offline) EXPLICIT HOST TERMINATION Cache has been deactivated by request from host system or support facility. DEACTIVATION IN PROCESS A command requesting deactivation of cache has been received, and destaging from cache to DASD is still in progress. DEACTIVATION FAILED A command requesting deactivation of cache has been received, but destaging to DASD has failed.
NON-VOLATILE STORAGE	Overall status of the non-volatile storage (NVS). ACTIVE NVS is online and usable. PENDING DUE TO ERROR A command requesting deactivation of NVS has been received but transfer from NVS to DASD has failed. DEACTIVATION IN PROCESS A command requesting deactivation of NVS has been received, and destaging to DASD is still in progress. DISABLED FOR MAINTENANCE NVS has been disabled for maintenance by the support facility. INTERNAL ERROR TERMINATION An internal error caused termination of NVS. EXPLICIT HOST TERMINATION NVS has been deactivated by request from host system or support facility. DASD FAST WRITE INHIBITED DASD FAST WRITE is inhibited because the battery is defective.
CACHE FAST WRITE	Status of the CACHE FAST WRITE (CFW) option. ACTIVE CFW is active. DEACTIVATED CFW is deactivated. Note: CFW does not use NVS.
IML DEVICE AVAILABLE	Status of the IML device. NO Device containing a diskette drive for loading the microcode is not available. YES Device is operational.

Table 3. Fields in the Cache Subsystem Activity Report - Subsystem Overview
Field Heading	Meaning
TOTAL I/O	Total number of I/O requests to cached devices in the storage subsystem.
TOTAL H/R	Ratio of I/Os that were processed within the cache (cache hits) based on the total number of I/Os.
CACHE I/O	Total number of cacheable I/O requests to cached devices in the storage subsystem. This value excludes INHIBIT CACHE LOAD and CACHE BYPASS I/O requests.
CACHE H/R	Ratio of I/Os that were processed within the cache (cache hits) based on the total number of cacheable I/O requests.
CACHE OFFLINE	Total number of I/O requests to non-cached devices in the storage subsystem.
CACHE I/O REQUEST - The channel command DEFINE EXTENT specifies the way the cache will be used. There are three categories (NORMAL, SEQUENTIAL, CFW DATA) and a TOTAL value:
NORMAL	Cache will be managed by least-recently-used (LRU) algorithm for making cache space available.
SEQUENTIAL	Tracks following the track assigned in the current CCW chain are promoted. They will be transferred from DASD to cache in anticipation of a short-term requirement.
CFW DATA	WRITE and READ-AFTER-WRITE requests are processed in cache. The data might not be written to DASD. Because CFW does not use the NVS, the application is responsible for restoring the data after a cache or subsystem failure.
TOTAL	This is either the sum of I/O requests, the total I/O rate, or the average hit ratio for the three categories described above.
READ I/O REQUESTS - Cache I/O requests that searched or read data from DASD. This is the number of channel operations that had at least one SEARCH or READ command but no WRITE commands. This is counted for cached devices only.
COUNT	Total number of SEARCH/READ requests.
HITS	Number of SEARCH/READ requests that completed without accessing the DASD.
H/R	Hit Ratio - number of SEARCH/READ hits compared to the total number of SEARCH/READ requests.
WRITE I/O REQUESTS - Cache I/O requests that wrote data to DASD. This is the number of channel commands that had at least one WRITE command. It is counted for cached devices only.
COUNT	Total number of WRITE requests.
FAST	Total number of DASD/CACHE FAST WRITE requests.
HITS	Number of DASD/CACHE FAST WRITE requests that completed without accessing the DASD (fast write hit).
H/R	Number of DASD/CACHE FAST WRITE hits compared to the sum of all READ and WRITE requests (excluding ICL and BYPASS).
%READ	Percentage of READ requests compared to the sum of all READ and WRITE requests (excluding ICL and BYPASS).
CACHE MISSES -Cache misses are calculated as the difference between total I/O requests and the number of cache hits. They are shown for normal, sequential, and CFW requests. A cache miss occurs if a record that is to be read from or written to a data set is not found in the cache. In either case, the track in which this record is located is searched on the DASD and transferred to cache. Subsequent requests referring to the same record can then be processed in cache only, provided that the data has not yet been moved out because cache space is needed for other data. Other reasons for cache misses are DFW bypass and DFW inhibit. In the first case, a lack of NVS caused the data to be written to DASD immediately, while in the second case, usage of DFW is inhibited (for example, the device does not allow DFW).
READ	Number of SEARCH/READ requests that needed access to DASD because the data could not be found in the cache.
WRITE	Number of WRITE requests that needed access to DASD because the data could not be found in the cache.
TRACKS	Number of tracks transferred from DASD to cache.
TOTAL	Total number of I/O requests that needed access to DASD because the data could not be found in the cache.
MISC - Miscellaneous cache activities
DFW BYPASS	Number of DASD FAST WRITE requests that would have resulted in a DFW hit; however, NVS was overutilized causing writes to be sent directly to DASD. This value is also known as DFW RETRY.
CFW BYPASS	Number of operations that did not transfer a track from DASD into cache because no free segments were available. If no free segments are available in cache, there is no destaging in favor of I/O requests with the CACHE FAST WRITE attribute. The I/O goes directly to the DASD.
DFW INHIBIT	If DASD FAST WRITE is active, this is the number of WRITE requests that inhibited DASD FAST WRITE. If DASD FAST WRITE is inactive, this is the number of WRITE requests that directly accessed the DASD, even with DASD FAST WRITE turned on.
ASYNC	Number of tracks transferred from cache to DASD asynchronously to transfer from the channel to release space in the cache and the NVS. For a duplex pair, this is the count of transfers from the cache to the secondary device of a duplex pair. Transfers from the cache to the primary device of a duplex pair are not counted. A high number of ASYNC I/Os is an indicator for an over-committed cache or NVS.
NON-CACHE I/O - READ requests that switched off cache processing.
ICL	Inhibit cache load. Number of I/O requests that inhibited load of data into cache although the data was not found in the cache. Note: If the data had been in the cache, it would have been counted as cache hit. Therefore, this is actually the number of ICL misses.
BYPASS	Number of I/O requests that explicitly bypassed the cache, irrespective of whether the data is in the cache or not.
TOTAL	Total number of I/O requests that bypassed the cache.
CKD STATISTICS - CKD (Count-Key-Data) is a format used to store data on DASD. The counts shown in this section are contained in the total write count.
WRITES	Number of write I/O requests in CKD format.
WRITE HITS	Number of write I/O requests in CKD format that could be resolved in the cache.
RECORD CACHING - Record caching is done dynamically upon a decision made by DCME or the microcode. It may improve overall cache performance if caching of whole tracks would waste cache storage. The decision is based on the number of I/Os, the hit ratio, and the locality of reference of a certain entity of data.
READ MISSES	Number of instances in which a record requested for READ was not found in the cache, and access to DASD was required.
WRITE PROM	Number of instances in which a record requested for WRITE was found in the cache, and access to DASD was not required.
HOST ADAPTER ACTIVITY¹⁾ - I/O activity of normal, sequential and CFW read and write requests.
BYTES/REQ	The average number of transferred bytes per read and write request.
BYTES/SEC	The average number of transferred bytes per second for read and write requests.
DISK ACTIVITY¹⁾ - Transfer activity from hard disk to cache and vice versa.
RESP TIME	Response time in milliseconds per read and write request.
BYTES/REQ	The average number of transferred bytes per read and write request.
BYTES/SEC	The average number of transferred bytes per second for read and write requests.
¹⁾ available for the IBM TotalStorage DS family

Duration reports: An asterisk behind a status field or the field VOLUME SERIAL indicates that the contents of the field has changed during the duration interval.