ABCs of z/OS System Programming Volume 3 - IBM Redbooks

Extended format data sets
A special type of this organization is the extended format data set. Extended format data sets
have a different internal storage format from sequential data sets that are not extended (fixed
block with a 32-byte suffix). This storage format gives extended format data sets additional
usability and availability characteristics, as explained here:
► They can be allocated in the compressed format (can be referred to as a compressed
format data set). A compressed format data set is a type of extended format data set that
has an internal storage format that allows for data compression.
► They allow data striping, that is, a multivolume sequential file where data can be accessed
in parallel.
► They are able to recover from padding error situations.
► They can use the system managed buffering (SMB) technique.
Extended format data sets must be SMS-managed and must reside on DASD. You cannot
use an extended format data set for certain system data sets.
z/OS UNIX files
Another type of this organization is the Hierarchical File System (HFS). HFS files are
POSIX-conforming files that reside in an HFS data set. They are byte-oriented rather than
record-oriented, as are MVS files. They are identified and accessed by specifying the path
leading to them. Programs can access the information in HFS files through z/OS UNIX
system calls, such as open(pathname), read(file descriptor), and write(file descriptor).
Programs can also access the information in HFS files through the MVS BSAM, QSAM, and
VSAM (Virtual Storage Access Method) access methods. When using BSAM or QSAM, an
HFS file is simulated as a multi-volume sequential data set. When using VSAM, an HFS file is
simulated as an ESDS. Note the following point about HFS data sets:
► They are supported by standard DADSM create, rename, and scratch.
► They are supported by DFSMShsm for dump/restore and migrate/recall if DFSMSdss is
used as the data mover.
► They are not supported by IEBCOPY or the DFSMSdss COPY function.
QSAM and BSAM
BSAM arranges records sequentially in the order in which they are entered. A data set that
has this organization is a sequential data set. The user organizes records with other records
into blocks. This is basic access. You can use BSAM with the following data types:
► Basic format sequential data sets, which before z/OS V1R7 were known as sequential
data sets, or more accurately as non-extended-format sequential data sets
► Large format sequential data sets
► Extended-format data sets
► z/OS UNIX files
QSAM arranges records sequentially in the order that they are entered to form sequential
data sets, which are the same as those data sets that BSAM creates. The system organizes
records with other records. QSAM anticipates the need for records based on their order. To
improve performance, QSAM reads these records into storage before they are requested.
This is called queued access. You can use QSAM with the following data types:
► Sequential data sets
► Basic format sequential data sets before z/OS V1R7, which were known as sequential
data sets or more accurately as non-extended-format sequential data sets
► Large format sequential data sets
► Extended-format data sets
► z/OS UNIX files
Chapter 4. Storage management software 155