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Choosing between Disk and Tape Storage

Under CMS, permanent SAS files can be stored in the following formats:

• disk format on disk

• sequential format on tape (Sequential format is commonly referred to as tape format.)

• sequential (tape) format on disk.

Advantages of Tape Format

Tapes are able to accommodate large files for which disk storage is impractical. In addition, CMS SAS files that are in tape format can be read by SAS under OS/390 and VSE (Version 7 tape format libraries cannot be read by SAS under VSE).

Disadvantages of Tape-Format Files on Tape

Tape-format files on tape have the following disadvantages:

• Accessing tapes is usually slower and more cumbersome than accessing files on disk because the tapes must be mounted by the operator.

• For nonlabeled tapes, you must position the tape to the correct file unless you are accessing the first file on the tape.

• Only one tape file can be accessed at a time. This means you cannot merge or concatenate two or more tape-format SAS data sets from the same tape. For example, this DATA step will not be executed:

        data new;
           set tape.one
              tape.two;
        run;

• If a SAS file on tape is modified or deleted, any file that follows (whether it is another SAS file or some other type of file) cannot be accessed.

• You cannot use the EDITOR procedure for SAS data sets that are stored in tape format because PROC EDITOR uses direct-access methods as opposed to sequentially processing one observation after another. Likewise, you cannot use SAS/FSP software's FSVIEW or FSEDIT procedures to process a tape-format data set.

• SAS data sets in tape format cannot be accessed randomly with the POINT= and NOBS= variables in the SET statement.

• There are no facilities to rename SAS files in tape format.

• Whether you are using tape or disk, each SAS file in tape format is stored as a single, physical file, as long as each file has a unique libref.

  Note:    If you write more than one SAS file in tape format using the same libref, they are written as one physical, sequential file.  

• If you replace or delete an existing SAS file that is in a sequential file with other SAS files, any SAS file that follows in the sequential file can no longer be accessed.

Disadvantages of Tape-Format on Disk

Tape-format files on disk have many of the same disadvantages as tape-format files on tape:

• You cannot use the EDITOR procedure for SAS data sets that are stored in tape format because PROC EDITOR uses direct-access methods as opposed to sequentially processing one observation after another. Likewise, you cannot use SAS/FSP software's FSVIEW or FSEDIT procedures to process a tape-format data set.

• SAS data sets in tape format cannot be accessed randomly with the POINT= and NOBS= variables in the SET statement.

• There are no facilities to rename SAS files in tape format.

• Whether you are using tape or disk, each SAS file in tape format is stored as a single, physical file, as long as each file has a unique libref.

  Note:    If you write more than one SAS file in tape format using the same libref, they are written as one physical, sequential file.  

• If you replace or delete an existing tape-format disk SAS file that is in a sequential file with other SAS files, any SAS file that follows in the sequential file can no longer be accessed.

CAUTION:

If you are using Version 5 sequential format, never use a libref or DDname that begins with TAPE for external files of any kind. See Working with SAS Files on Tape for more information about tape handling under CMS.  

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