This command displays cache usage within the storage groups associated with the storage class used to dynamically allocate space for Virtual Volume data sets. If the optional command parameter is not specified and the total space defined in the storage groups used for the CA Vtape cache is less than one terabyte, the space will be displayed in megabytes. If the space is equal to or greater than one terabyte, the display will be in gigabytes.
SVTn Display Cache [,{Gb | Mb}]
Specifying the optional command parameter Gb displays all cache values in gigabytes regardless of the total space defined in the storage class.
Specifying the optional command parameter Mb displays all cache values in megabytes regardless of the total space defined in the storage class.
Example of information displayed by the DISPLAY CACHE command
SVT7 D C SVT7X2004I Cache Information Display 199 Cache free space threshold... 90% Cache DSN prefix............. SVTS120.VT3 Dynamic cache (SMS constructs defined in Global): Data class............... DCLOANV Storage class............ SCLOANV Dynamic cache (SMS constructs returned by ACS): Data class............... DCLOANV Storage class............ SCLOANV Storage Groups MB.Alloc MB.Free MB.InUse Use% -------------------------------------------------------------------- SGLOANV1 75,790 29,646 46,144 60% -------------------------------------------------------------------- Total: 16 cache volumes 75,790 29,646 46,144 60% Useable Storage Recap MB.Alloc MB.Free MB.InUse Use% -------------------------------------------------------------------- Dynamic cache space 64,269 25,139 39,130 60% Not Freeable n/a n/a 27,454 42% 7 Unexpired Volumes n/a n/a 9,175 14% 5 Expired Volumes n/a n/a 2,501 3% -------------------------------------------------------------------- Total: Cache utilization 64,269 25,139 39,130 60% SVT7X0100I Command Complete
The Storage Groups section of the display is composed of totals determined by using an IDCAMS DCOLLECT for the CA Vtape Storage Group. The amounts returned are track-based or raw storage totals.
The Useable Storage Recap section of the display is composed of totals determined by taking the track-based amounts and adjusting them for the actual VSAM Linear Data Set (LDS) allocations. The Dynamic LDSs are allocated in 16 KB blocks. Three blocks will fit on each 3390 DASD track for a total of 49512 bytes. Because each track is 56664 bytes in size, an LDS only uses 86.7% of the track ((49512/56664) * 100).
Partial space-release also requires an adjustment to the track-based amounts. Idle space can be released only at the VSAM Control Area (CA) boundary. Only empty CAs can be released.
The LDSs have a CA of one cylinder or 15 tracks. If one data record is written to a CA, the 14 empty tracks cannot be released. Testing has determined that unreleased, empty tracks account for approximately another 1.9% of the available space.
Adding these two factors indicates that available space should be adjusted to 84.8% to ensure that adequate free space is maintained to support virtual mount activity. By internally adjusting the cache figures, CA Vtape triggers the CacheFreespaceThreshold, CacheWarningThreshold, and CacheAutoReleaseHighThreshold actions before a true or operational cache shortage can occur. Relying on the track-based amounts could cause the thresholds to be triggered too late.
The adjustment will not cause Dynamic Cache Management to require more DASD volumes than Static Cache Management. Dynamic Cache Management increases DASD storage usage per volume, which should reduce the total amount of DASD required to keep the same quantity of Virtual Volumes resident in cache for the same period of time.
Not Freeable indicates the amount of dynamic cache space being utilized by Virtual Volumes that are yet to be Externalized or are in use as well as any other non SVTS space. Unexpired Volumes indicates the amount of dynamic cache space being utilized by Virtual Volumes that have been Externalized but that have not reached their cache expiration date (not to be confused with tape expiration date). Expired Volumes refers to Virtual Volumes that have been Externalized and have also reached or exceeded their cache expiration date.
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