If you save a fixed/variable/undefined record length MVS data set in text mode with one or more lines exceeding the maximum record length, an I/O error occurs. For example, suppose an MVS data set has fixed-length records of 80 bytes. After you edit the file using the vi editor on your workstation, one of the file’s records is 83 bytes long (exceeding the fixed length by 3 bytes). When you save the file back to the server, the MVS data set may be either partially or totally destroyed, and the "I/O Error" message appears on your screen. While you are still in the editing session, save the edited file in an alternate local file. After you correct the local file so that no line exceeds 80 bytes, save it back into the MVS data set.

For example, suppose a variable-length MVS data set is defined with an LRECL of 132 bytes. The maximum effective record length of the data set is actually 128 bytes, because 4 bytes are reserved by DFSMS SAM to accommodate the record descriptor word (RDW). As a result, any line that is attempted to be written into this data set that exceeds this maximum effective record length will result in an I/O error.

For more information about I/O errors related to different MVS data set types, see z/OS DFSMS Using Data Sets