VSAM R/M uses two types of control blocks:

• User control blocks (RPL, ACB, and EXLST)
• Control blocks built for record management by VSAM OPEN (all other control blocks).

The RPL's RPLFDBWD field (4 bytes at offset X'0C') provide valuable failure-related codes. For descriptions of the return codes and reason codes, see Understanding VSAM macro return and reason codes.

The VSAM SNAP dump facility provides hexadecimal listings of some control blocks and data areas. To obtain a SNAP dump, perform one of the following tasks:

• Code the SNAP macro with the SDATA=CB option (described in z/OS MVS Programming: Authorized Assembler Services Reference SET-WTO).
• Specify this option to ABDUMP via the CHNGDUMP operator command or the IEAABDnn or IEADMPnn members of SYS1.PARMLIB.

The SNAP dump facility is only available at SYSABEND time.