09 compute vsam file space requirements
How to Compute VSAM File Space Requirements
The size of the EXPO and EARS VSAM files can vary dramatically from State to State. As a result, the guidelines for VSAM File CICS definitions (outlined earlier in this Appendix) will have vastly different values in the District of Columbia than would be needed in the State of Texas. The size of VSAM-indexed files is not a clear and simple computation, since such factors as free-space percentage, control interval size versus record size, record key size, and potential file fragmentation, must enter into the overall considerations.
The formulations that follow provide a concise, rule-of-thumb method of computing maximum disk space requirements of all of the EARS and EXPO system indexed files. In each equation, the value listed is the amount of disk space to allocate for the file to make the best usage of disk resources while still having sufficient space for the data. Usually these are noted in “cylinders” (where a cylinder is a parallel track on each of the fifteen disk surfaces available at a specific distance from the edge of the disks). A “track” refers to one “ring” of data on one of the disk surfaces. There are eight disk platters in a disk pack, with both surfaces of each accessible to a reader head, except for the bottom disk, which is on the floor of the drive compartment.
A total of about 760,000 characters can be stored on one cylinder, so a track (1/15 of a cylinder) amounts to about 51,000 characters. The total capacity of an entire disk pack is around 3,800,000,000 characters. About 15 years ago, that would have been considered a large capacity. But, with the advent of ever cheaper, ever-higher-capacity hard disks for PC’s, this has become practically miniscule. PC hard disks in the early 1990’s would cost about $1 per megabyte of storage capacity. In 2006, the cost is about $0.20 per gigabyte (a 99.98% price reduction). So the storage capacity of about $2,000,000 worth of mainframe disk packs can reside in a $50 PC hard disk.
Getting back to the subject at hand: The total allocation space of a file is listed for those States that have no allowance for file allocation extents. This is a rare condition, however, since most States will use “extents.” In this condition, an initial base allocation is assigned, with smaller, supplemental values used for the extents (for “extensions” or supplements) to the file allocation. To set these up, the following equations should be used:
Otherwise, the Total Allocation should be used as the base value. In this instance, no extent should be listed; if it is given, it should be set to zero (0), keeping the file space constant. The primary benefit of using extents is that the utilization of disk space is more optimally served. A non-expandable file must be established with the largest imaginable space reserved, so that the rest of that disk space (which remains constantly unused) will be kept in reserve. Sometimes billing for hard disk space can make such a practice a costly one. The base/extent method can save money.
In determining the base allocations, the following parameters need to be filled in for the specific State. These will reduce the file size computations to a set of simple equations. They include:
- MC = Micro Record Count (the number of establishments that will exist on the Micro File -- active, inactive, and pending). Unit = thousands of records.
- CC = County Code Count (the number of distinct FIPS county codes used in the State).
- MAY = Number of years of macro-level data to store in the Macro File (minimum of 2).
- TFSD = Transaction File Storage Days (the number of business days (five per week) for which the Administrative and Quarterly Transaction Files are allowed to grow before being purged to keep their size under control). Most States will use 125 for this value (the approximate number of business days in a six-month time span.
The only two EARS files to be defined are the Refiling File and the Refiling Transaction File. The size of the Refiling File is computed from:
The Refiling Transaction File is more a matter of guesswork, as it is difficult to know how many transactions, on average, each refiled establishment will experience over the course of the year. The initial formulation, which may require adjustment as this file becomes more recognized, is:
The size of the Alpha Locator File is found from:
The Administrative Transaction File space requirements can be found from:
The Account Groups File size is determined via:
The Lookup File maximum size is constant, at 3 cylinders or 45 tracks.
The Macro File maximum expansion is expressed as:
Remember that “MAY” is the number of years of data to be stored on the Macro File.
The Macro Edit File size can be measured in tracks, as shown:
The Micro Edit Cross Reference (MEXR) File’s size is found via:
The Micro File size is computed from:
The Micro Edit File space requirement is found with:
The Micro Omni Auxiliary File size would be:
The MOA Transaction File size is found through:
The Narrative Comments File space is determined from:
The Predecessor Successor Actual (PSA) File requires the following disk space allocation:
The Predecessor Successor Narrative (PSN) Notes File space needs are found via:
The Predecessor Successor Potential (PSP) File may not be developed by all States. For anyone using this file, the space needed can be found from:
The Publications Parameter Specifications File space allocation varies based on the number of publications the State produces. In general, though, it may be set to 5 cylinders for all States.
The Quarterly Transaction File space requirements can be found from:
The Rollup Parameters File size is fixed, at 2 cylinders.
The Internal Security File size is also fixed, at 2 tracks (except at the multi-state Service Center, where it is set to 8 tracks).
The Statewide Macro File size is found from the equation:
The size of the Wage Summary File, if it is used, is computed via:
Finally, the space needed for each of the two Crosswalk Files is established as:
The county-based Crosswalk File (“XWLK”) requires more space than the statewide Crosswalk File (“XWK2”). The reason for this additional space is that an additional set of records are included in the county file, namely the EIN-to-account number crosswalk records.
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