Migration Considerations
Disk Formats
format has a completely revised internal file structure. The extended format struc­
ture is highly compatible with previous versions of eMS and retains the strengths
of earlier eMS file systems.
The VM!SP file system provides the following for extended format disks:
The logical disk capacity has been increased. There is no effective limit to the
size of a eMS minidisk except for the size of the physical device.
The logical file capacity has been increased. There is no effective limit on the
size of a file except for the amount of space on the minidisk. (The record
number must be less than 2
31
_1.)
Performance when randomly accessing variable length records has been
improved.
The number of minidisks that can be accessed at anyone time by a single user
has been increased from 10 to 26.
In VM/370, the number of files per minidisk was limited to 3400. With the VM/SP file system extensions, there is no effective limit except for the con­
straints of storage and disk space.
In VM/370, the minimum number of physical disk blocks needed to hold very
small files was two; it is now one. Note that this mayor may not cause a sav­
ing of disk space depending on the physical blocksize chosen and the exact size
of the small file.
Internal algorithms controlling the updating of blocks on disk when files are
closed have been redesigned. The redesign provides a significant increase in
performance for most eMS users.
The physical blocksize of the VM/370 file system was 800. The VM/SP file system supports four additional sizes:
512 bytes 1024 bytes 2048 bytes 4096 bytes
The in-storage control structures of the VM/SP file system are considerably differ­
ent than those of the VM/370 800-byte file system. However, because backward
compatibility has been maintained in the VM/SP file system, disks that are format­
ted under earlier versions of eMS can be used with VM/SP. Disks that are for­
matted with a blocksize of 800-bytes under VM/SP can be used with earlier
versions of eMS. Note, however, that the default blocksize for VM/SP is 1024 bytes when minidisks are formatted using the VM/SP FORMAT command.
Although the internal block size is transparent to most users and programs, installa­
tion utilities that dump and restore disks may depend upon the physical disk
blocksize and the internal disk control block structure.
Introduction To eMS 305

MACLIB and TXTLIB Files
TAPE Command
DISK Command
Program I/O The internal format of MACLIB and TXT LIB files has been augmented by the
addition of a new format to allow larger libraries. Earlier CMS library formats are
supported by VM/SP whether they exist on 800-byte or extended format disks.
Updating an old format MACLIB or TXT LIB on an extended disk does not change
the internal format. The creation of a library on an extended format disk causes
the construction of a library with the new format.
Under VM/SP, new format libraries are supported on 800-byte format disks. This
condition can only occur if a new format library is copied (via COPYFILE, MOVEFILE, and so on) from an extended format disk to an 800-byte format disk.
Note, however, that the new format libraries are not supported by earlier versions
of CMS even if on 800-byte disks.
The format of tapes created by the TAPE command has been augmented by the
addition of a larger blocksize of 4096-bytes. Tapes created by earlier versions of CMS are properly read onto any disk format by VM/SP. Tapes created by VM/SP are not readable by earlier versions of CMS unless they are dumped with
blocksize of 800. The default blocksize is 4096 when minidisks are dumped using
the VM/SP TAPE command.
The format of spool files created by the DISK command differs slightly from earlier
versions of CMS. However, files dumped by previous versions of CMS are proper­
ly read by VM/SP and files dumped by VM/SP are properly read by earlier ver­
sions of CMS provided that the file meets the size constraints of the 800-byte disk
(especially, the dumped file must not be greater than 65,533 records).
Programs that do I/O to CMS disks fall into three categories:
Those that do CMS I/O (for example, FSOPEN, FSREAD, FSWRITE)
Those that do OS I/O (for example, OPEN DCB, READ, GET) • Those that do VSE I/O (for example, OPEN DTF, READ, GET) Only programs that do CMS disk I/O directly have any compatibility or migration
considerations. Programs that issue OS or VSE I/O calls can immediately take
advantage of the capacity of the extended file system as soon as the files are put on
a VM/SP extended format disk.
Programs that issue CMS I/O macros or calls continue to work on both the 800-byte and the VM/SP extended file systems but are not able to take advantage
of all of the VM/SP file capabilities without conversion. This includes the use of
the FSST ATE macro, which returns the correct format File Status Table (FST) whatever the disk format.
However, it should be noted that, in general, programs that deal with internal sys­
tem control blocks, (such as File Status Table (FST) blocks, Active Disk Table
(ADT) blocks, or Active File Table (AFT) blocks) should not be used under VM/SP without careful examination of the program, and conversion or elimination
of program references to the internal blocks. 306 VM/SP System Programmer's Guide