COMPAKTOR VTOC Errors

As previously noted, COMPAKTOR performs rather stringent error diagnostics on VTOCs. Problems that exist but are not diagnosed by any other software are detected and reported by COMPAKTOR. Our experience indicates that these are the most common errors:

Invalid F5/F7 DSCBS

Format 5 and 7 DSCBs describe the free space on a volume with no indexed VTOC (F7s are used on large volumes such as 3390-9s). There generally are two types of F5/F7 DSCB errors: either the free space described is not valid (starting or ending track address is not valid), or a free area described is actually allocated to a current data set. The first case leads to loss of free space, but is generally non-harmful; the second case, however, is extremely serious and can lead to loss of data if the space is assigned to a second data set.

Important

If an active indexed VTOC exists on a volume, then the F5/F7 DSCBs are never valid, and CPK does not inspect them.

Overlapping extents

Overlapping extents occur when two or more F1 or F3 DSCBs define extents that contain the same tracks. This is an extremely serious condition and can lead to loss of data.

Invalid extent descriptors

An extent descriptor in an F1 or F3 DSCB is invalid if its cylinder and track addresses are invalid for the device or the extent ending address is lower than the starting address. Such a data set may be unusable.

Invalid flags in extent descriptors

Each extent descriptor has a flag byte, describing the type of extent. A common error is that the flag byte defines the extent as being on a cylinder boundary when it is not. This error may or may not be serious, depending on data set organization and access techniques used.

Broken F1-F2-F3 DSCBs

Format 2 DSCBs exist for ISAM data sets; Format 3 DSCBs exist for any type of data set that has more than three extents. The Format 1 DSCB points to the Format 2 or Format 3 DSCBs if they exist (a Format 2 may point to a Format 3). COMPAKTOR detects any broken chains, as well as any unrelated F2/F3 DSCB; that is, when an F2/F3 DSCB is present but its F1 DSCB is missing. This condition is serious if a broken chain exists, but harmless if an unrelated DSCB exists. However, the error must be corrected for COMPAKTOR to run.

See member $$CPK in the FDR Installation Control Library (ICL) for information on correcting DSCB chains.

Format 4 DSCB errors

A warning message is issued if error flags are found set in the VTOC status byte field of a Format 4 DSCB. In addition, if the F4 DSCB is not the first record in a VTOC, COMPAKTOR cannot process the volume; the volume violates IBM Operating Systems standards.