CACHE (Disk Cache)

The CACHE (Disk Cache) system command controls the cache memory status of disks, displays cache statistics for the entire system, and dynamically resizes the amount of memory allocated to the disk cache.

Syntax

── CACHE ─┬─────────────────────────┬─────┤
          ├─ + ─┬─ PK ──<unit list>─┤
          ├─ - ─┘                   │
          └─ MEMORY ──<cache size>──┘

<cache size>

─┬─ <megaword limits> ─┬─────────────┤
 └─ <gigaword limits> ─┘

<megaword limits>

── <integer between 4 and 262144> ─┬─ MEGAWORDS ─┬──────┤
                                   └─ MWORDS ────┘

<gigaword limits>

── <integer between 1 and 256> ─┬─ GIGAWORDS ─┬───────┤
                                └─ GWORDS ────┘

Explanation

CACHE

Displays the following information:

  • The number of disk units that are currently under the control of the disk caching subsystem

  • The ratio of disk cache read hits to cache requests for all cached disks within the last sampling interval

  • Disk cache memory size

  • The total number of cache requests for all cached disks within the last sampling interval

  • The overall data transfer rate of data for all cached disks within the last sampling interval

  • Traffic statistics for each cached disk within the last sampling interval

CACHE + PK <unit list>

Starts caching for the specified units.

CACHE − PK <unit list>

Stops caching for the specified units. This command also stops maintaining the disk cache statistics for the specified units.

CACHE MEMORY <cache size>

Dynamically resizes cache memory.

If <cache size> is less than the current size of the memory allocated to the disk caching subsystem, the amount of memory allocated to the disk caching subsystem is decreased to <cache size>. Setting the <cache size> to zero suspends disk caching.

If <cache size> is greater than the current size of memory allocated to the disk caching subsystem, the amount of memory allocated to the disk caching subsystem is increased to <cache size>.

When increasing the disk cache, the MCP does not allow the system memory size to fall below the minimum size defined for the system.

The resizing of disk cache memory usually takes place immediately. Under some circumstances, however, areas marked as "save memory" can prevent the size of disk cache memory from being increased. In this case, a halt/load is required for the request to occur. As a result, a waiting entry appears with the following RSVP: 

Unable to resize disk cache memory. Responses are DS (Give up),
AX DEFER (Resize cache next haltload), or AX HALTLOAD (Haltload 
and resize cache now).

Examples

Example 1

CACHE
  Disk Caching Statistics:
  Caching Packs = 35
  Overall ReadHits/Requests:76% Cache Size:10 Mwords
  Cache:          70 Reqs/Sec    336 Kbytes/Sec
                          %      %       %
                 Reqs/ Rhit/  Read/   Rhit/   Kbytes/
  PK              Sec   Read    All     All    Sec
  165 SYSTEM #1    2     75      23      17      5
  166 CAE #1       1    100      44      44      1
  176 DISK #1      6    100      49      49      5
  184 PACK #1      3     23      96      22     38
  185 X3 #1        3     94      72      67      7
  187 BDRIVE #1   55     97     100      97    280

The CACHE command displays the following items:

  • Caching Packs

    The number of online packs you are currently caching. Each cached mirrored set counts as one pack, and packs that are closed or are without a path are not counted.

  • Overall ReadHits/Requests

    The percentage of cache requests that were read hits.

  • Cache Size

    The size of the disk cache. Mwords  indicates that the size is in megawords.

  • Reqs/Sec

    The average number of cache requests per second.

  • Kbytes/Sec

    The average number of kilobytes per second requested by cache requests.

Unit Statistics

The rest of the displayed information is unit statistics. In order for a unit to be included in the display, at least one cache request must have been entered for that unit during the preceding time interval. Inactive units and noncached units are not included in this display.

  • Reqs/Sec

    The average number of cache requests for this unit per second.

  • % Rhit/Read

    The percentage of read requests for this unit that were read hits.

  • % Read/All

    The percentage of all requests for this unit that were reads.

  • % Rhit/All

    The percentage of all requests for this unit that were read hits. This column is always equivalent to the product of the previous two columns.

  • Kbytes/Sec

    The average number of kilobytes per second requested by cache requests for this unit.

Example 2

This example turns on caching for disk pack 149:

CACHE + PK149
  PK 149 CACHING STARTED

Example 3

This example turns off caching for disk pack 149:

CACHE - PK149
   PK 149 CACHING STOPPED

Example 4

This example resizes cache memory to 16 megawords:

CACHE MEMORY 16 MWORDS
   CACHE MEMORY WILL BE RESIZED
   CACHE MEMORY RESIZED TO 16 MWORDS
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