IBM OS/390 User Manual
5.5 DASD Similarities and Differences
5.5.1 Volume Interchangeability
DASD file label conventions, requirements, and handling techniques differ
between VSE and OS/390 systems. However, OS/390 should be able to read and
process DASD volumes that have been created on a VSE system. Similarly, VSE
systems should be able to read volumes created by OS/390 systems. The
following exceptions to this are noted below.
between VSE and OS/390 systems. However, OS/390 should be able to read and
process DASD volumes that have been created on a VSE system. Similarly, VSE
systems should be able to read volumes created by OS/390 systems. The
following exceptions to this are noted below.
•
OS/390 doesn
′
t support FBA (Fixed Block Architecture) DASD devices; for
example, 3310, 3370, 9332, 9335, 9336. Data files located on FBA devices will
have to be copied to an OS/390-supported device using a VSE
backup/restore program.
have to be copied to an OS/390-supported device using a VSE
backup/restore program.
•
Concurrent sharing of a volume between the two systems is not supported
and should not be attempted as data loss could result.
and should not be attempted as data loss could result.
•
OS/390 does not allow more than one volume with an identical volume serial
number (VOLID) to be online at any one time. VSE would allow this condition.
Thus, if you are used to operating with identical serial number volumes
(either tape or DASD) mounted concurrently, you will have to change to
unique volume serial number identifiers for OS/390 operations.
number (VOLID) to be online at any one time. VSE would allow this condition.
Thus, if you are used to operating with identical serial number volumes
(either tape or DASD) mounted concurrently, you will have to change to
unique volume serial number identifiers for OS/390 operations.
•
It is not recommended that OS/390 volumes which contain Indexed VTOCs be
accessed by a VSE system. To do so requires special procedures - See 5.5.3,
“Indexed VTOC Considerations (OS/390)” on page 109.
accessed by a VSE system. To do so requires special procedures - See 5.5.3,
“Indexed VTOC Considerations (OS/390)” on page 109.
5.5.2 DASD (VTOC) Processing
DASD volumes are managed by both VSE and OS/390 through a Volume Table of
Contents (VTOC) which is a type of file. There is always one VTOC per volume.
Each VTOC is composed of various records called Labels in VSE, but named
Data Set Control Blocks (DSCBs) in OS/390. DSCBs are used to store information
about the contents of the volume. DSCBs come in various record formats with
varying field layouts, and are customized by the type of information stored.
There can be up to six different DSCB types in a VTOC. They are appropriately
named:
Contents (VTOC) which is a type of file. There is always one VTOC per volume.
Each VTOC is composed of various records called Labels in VSE, but named
Data Set Control Blocks (DSCBs) in OS/390. DSCBs are used to store information
about the contents of the volume. DSCBs come in various record formats with
varying field layouts, and are customized by the type of information stored.
There can be up to six different DSCB types in a VTOC. They are appropriately
named:
″
Format-1 DSCB ,..., Format-6 DSCB
″.
For a detailed description of a
VTOC layout, index VTOC and the various DSCBs, see the
DFSMSdfp Advanced
Services. Also, see VSE/AF Data Management Concepts , GC33-6192. For
purposes of this publication, only the Format-1, Format-4, and Format-5 DSCB
need to be discussed.
purposes of this publication, only the Format-1, Format-4, and Format-5 DSCB
need to be discussed.
Both VSE and OS/390 use the Format-1 DSCB. There is a Format-1 DSCB record
for each data set allocated on a volume. OS/390 stores information about each
data set in addition to the information which VSE stores. As such, VSE does not
maintain all fields in the Format-1 DSCB. For example, the record format, record
length, and block size are not maintained by VSE. Thus, to access VSE-created
files from an OS/390 system, the user must supply (preferably through JCL) file
information which otherwise would be obtained from the file
for each data set allocated on a volume. OS/390 stores information about each
data set in addition to the information which VSE stores. As such, VSE does not
maintain all fields in the Format-1 DSCB. For example, the record format, record
length, and block size are not maintained by VSE. Thus, to access VSE-created
files from an OS/390 system, the user must supply (preferably through JCL) file
information which otherwise would be obtained from the file
′
s Format-1 label or
DSCB.
Both VSE and OS/390 use a Format-4 DSCB -- there is one (and only one) per
VTOC. The Format-4 DSCB contains a 1-bit flag designator (called the
VTOC. The Format-4 DSCB contains a 1-bit flag designator (called the
″
DOS bit
″)
which is used to indicate whether the Format-5 DSCB(s) are valid. Turning the bit
″
on
″
or
″
off
″
is the responsibility of the operating system. VSE when it first uses
a volume turns the
″
DOS bit
″ ″
on
″.
The use of this bit is mainly for the purpose of
108
VSE to OS/390 Migration Workbook