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The VTOC Index z/OS DFSMSdfp Advanced Services SC23-6861-01 |
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The VTOC index enhances the performance of VTOC access. The VTOC index is a physical-sequential data set on the same volume as the related VTOC. It consists of an index of data set names in format-1 DSCBs contained in the VTOC and volume free space information. An SMS-managed volume requires an indexed VTOC; otherwise, the VTOC index is highly recommended. For additional information about SMS-managed volumes, see z/OS DFSMS Implementing System-Managed Storage. Note: You can use the ICKDSF REFORMAT REFVTOC command
to rebuild a VTOC index to reclaim any no longer needed index space
and to possibly improve access times.
z/OS® does support sharing a non-SMS-managed volume that contains a VTOC index with a non-z/OS system. If the other system updates the VTOC and turns on the DS4DOSBT, then later when z/OS is used to modify the VTOC, DADSM can detect that the index is no longer valid. The z/VSE® operating system sets this bit on. Device Support Facilities (ICKDSF) initializes a VTOC index into
2048-byte physical blocks, or 8192-byte physical blocks
on an extended address volume, named VTOC index records (VIRs). The
DEVTYPE INFO=DASD macro can be used to return the actual block size
or it can be determined from examining the format-1 DSCB of the index
data set. VIRs are used in several ways. A VTOC index contains
the following kinds of VIRs:
A format-1 DSCB in the VTOC contains the name and extent information
of the VTOC index. The name of the index must be 'SYS1.VTOCIX.volser',
where 'volser' is the serial number of the
volume containing the VTOC and its index. The name must be unique
within the system to avoid ENQ contention and must conform to standard
data set naming conventions.
Note:
You can only create (allocate) one data set whose name begins with 'SYS1.VTOCIX.' on a volume. To rename a VTOC index data set when the VTOC index is active, use a name beginning with 'SYS1.VTOCIX.'. If a 'SYS1.VTOCIX.' data set already exists on a volume, you cannot rename another data set on the volume to a name with those qualifiers. If the VTOC index is active, you cannot scratch the VTOC index data set. The relationship of a VTOC to its index is shown in Figure 1. Figure 1. Example of the Relationship
of a VTOC to Its Index
VTOC VTOC Index ┌────────────────────────────┐ ┌────→┌─────────────────────────┐ ³ Format─4 DSCB ³ ³ ³ VIXM ³ ├────────────────────────────┤ ³ ├─────────────────────────┤ ³ Format─5 DSCB ³ ³ ³ VPSM ³ ├────────────────────────────┤ ³ ├─────────────────────────┤ ³ ³ ³ ³ VMDS ³ ³ Other DSCBs ³ ³ ├─────────────────────────┤ ³ ³ ³ ³ VIER ³ ³ ³ ³ ├─────────────────────────┤ ├────────────────────────────┤ ³ ³ VIER ³ ³ Format─1 DSCB for the VTOC ├─────┘ ├─────────────────────────┤ ³ Index: SYS1.VTOCIX.nnn ³ ³ VIER ³ ├────────────────────────────┤ ├─────────────────────────┤ ³ ³ ³ . ³ ³ Other DSCBs ³ ³ . ³ ³ ³ ³ . ³ └────────────────────────────┘ └─────────────────────────┘ |
Copyright IBM Corporation 1990, 2014
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