Skip to main content

IBM Systems  >   System z  >   z/VM  >  

The Anatomy of the z/VM Volume Table of Contents

The Volume Table of Contents (VTOC) on a CP-owned volume (CPVOL) is a bit unusual. The purpose of the VTOC is to tell z/VSE and z/OS, both of which use the VTOC to manage space on the volume, that there is no room on the volume. They can look, but not touch.

Here's the format of the VTOC. The VTOC is pointed to by the label. Volume Table of Contents (VTOC)

Now let's look at a real example. This 10-cylinder disk was formatted using CPFMTXA. It has a volume label of TEMPAA.

Label

The label is always cylinder 0, track 0, record 3, and is at least 80 bytes in length. CP-owned volumes use an IBM standard 80-byte VOL1 label. 

Rec Type  Len Byte ---------------- Hex -----------------   ------ Char ------
3  DATA    80 0000 E5D6D3F1  E3C5D4D7  C1C1F000  00000005   *VOL1TEMPAA0.....*
              0010 00000000  00404040  40404040  40404040   *.....           *
              0020 40404040  40404040  40000000  0000C3D7   *         .....CP*
              0030 E5D6D340  40404040  40404040  40404040   *VOL             *
              0040 40404040  40404040  40404040  40404040   *                *

TEMPAA is the 6-byte volume label (volser).

5X'00',CL5'CPVOL' is a magic cookie that indicates this is a CP-owned volume. That means that it contains extent description data elsewhere on cylinder 0. CP will not read the allocation map from a volume that doesn't contain the magic cookie.

0000 0000 05 is the CCHHR (5-byte disk address) of the VTOC. This means cylinder 0, track 0, record 5. The VTOC on a CP-owned volume is always at this location. (Since there is one head per track, the terms head and track are interchangeable.)

The VTOC on a CPVOL contains exactly two records:

  1. A format-4 Data Set Control Block (DSCB-4) that describes the volume, and
  2. A format-5 Data Set Control Block (DSCB-5) that describes the one and only usable extent on the volume.

VTOC DSCB-4 (volume description)

The DSCB-4 is always 140 bytes in length. It contains information about the volume, the device geometry at the time of formatting, and the size of the VTOC. The first 44 bytes are the key. 
Rec Type  Len Byte ---------------- Hex -----------------   ------ Char ------
5  KEY     44 0000 04040404  04040404  04040404  04040404   *................*
              0010 04040404  04040404  04040404  04040404   *................*
              0020 04040404  04040404  04040404             *............    *
 
   DATA    96 0000 F4000000  00050000  00000000  00000001   *4...............*
              0010 0000000A  000FE5A2  00000030  0000322D   *......Vs........*
              0020 00000000  00000000  00000000  00000000   *................*
              0030 00000000  00000000  00000000  00000000   *................*
              0040 00000000  00000000  00000000  00000000   *................*
              0050 00000000  00000000  00000000  00000000   *................*

0000: The VTOC is full. No more DSCBs can be allocated within it.

00: The VTOC is not indexed.

000A: Number of cylinders on the volume. This number may be greater than the number of formatted cylinders.

Buried in there is also the information that there are 15 tracks per cylinder and 58 786 bytes per track.

Since the VTOC is simply a defensive mechanism that isn't actually used by CP, there's no issue if you copy the disk contents onto a larger volume. However, the mismatch in information in the VTOC and the size of the real volume could upset z/OS or z/VSE. Beware.

VTOC DSCB-5 (free space)

The DSCB-5 is always 140 bytes in length and describes the (logically) available space on the volume. The first 44 bytes of this DSCB are the key, though it isn't needed since it always follows the DSCB-4 and can be read at the same time. See the picture above.

The DSCB-5 always says

  • The first extent that MAY contain free space starts on track 1.
  • There are no available cylinders or tracks in the extent starting on track 1.
  • There are no other extents on the volume.

So it doesn't really matter where this extent of (alleged) free space is located - it's full. 

Rec Type  Len Byte ---------------- Hex -----------------   ------ Char ------
6  KEY     44 0000 05050505  00010000  00000000  00000000   *................*
              0010 00000000  00000000  00000000  00000000   *................*
              0020 00000000  00000000  00000000             *............    *
 
   DATA    96 0000 F5000000  00000000  00000000  00000000   *5...............*
              0010 00000000  00000000  00000000  00000000   *................*
              0020 00000000  00000000  00000000  00000000   *................*
              0030 00000000  00000000  00000000  00000000   *................*
              0040 00000000  00000000  00000000  00000000   *................*
              0050 00000000  00000000  00000000  00000000   *................*

0001: Track offset to the first extent with free space.

0000: No unused cylinders in the extent.

00: No unused tracks in the extent.

All the other extents in this DSCB-5 (there are 26 total) are zero, as is the pointer to the next DSCB-5 in the VTOC. Why isn't the first one zero? The world may never know....

Details about these disk structures can be found in the following z/OS manuals:
  • DFSMS: Using Data Sets, SC26-7410, Appendix A (IBM VOL1 standard labels)
  • DFSMSdfp Advanced Services, SC26-7400, Chapter 1 (VTOC and DSCBs)

I hope this helps anyone who needs to understand the VTOC. I also place it here for posterity.