Page of GC20-1801-10 As Updated April 1, 1981 by TNL GN25-0837 VM/370 Storage Requirements
Estimating VM/370 Storage Requirements
This section contains information about: • Estimating real storage requirements for VM/370 • Reducing the size of the CP nucleus • Estimating direct access storage requirements • Estimating storage requirements for CMSminidisks The "Specifying a virtual=Real Machine" section includes information
about estimating real storage requirements for a virtual=real machine.
Note: The requirements specified here are applicable only to the SCP, not to its extensions. Real Storage Requirements for CP
Figure 14 lists the various CP requirements and the amount of real
storage required for each.
r----- CP Requirement
Res iden t nu cleus
Internal trace table
Real control blocks
Permanently allocated
free storage
(virtu al cont rol
blocks and tables).
For installation
control of free
storage, use the SYSCOR macro. See "Part 2. Defining Your VM/370 System." Real Storage Allocated
Approximately 152K
Conventionally, 4K of storage is allocated
for each 256K of real storage. This
storage is set aside at IPt time. See "SYSCOR Macro" in Part 2 for details of
how to increase the size of the internal
trace table.
There is a control block for each real
dev ice, control unit, and channel: • 88 bytes/real device • 72 bytes/real control unit • 96 bytes/real channel • 24 bytes for each remote 3270 or real 3704/3705 The default value is a minimum of 12K, plus
an additional 4K for each 64K of real
storage above 256K.1 This storage is set
aside at IPL time. Each logged-on virtual
machine requires a virtual machine ,control
block (VMBLOK), a segment table (SEGTABLE), a page table (PAGTABLE), a swap table (SWPTABLE), and a control block for each
virtual device, control unit, and channel.
igure 14. Real Storage Requirements for CP Requirements (Part 1 of 2) IAn additional 25% of free storaqe is allocated in AP mode. Part 1. Planning for System Generation 87

-- ---v . -_ .. - ... "r-- .... -"""'" ..... " ,J...." "'J ......... u" ..... .,J V'J..J' VM/370 storage Requirements
L CP Requirement Real storage Allocated
The storage required is: • 504 bytes for the VMBLOK • 64 bytes/1M of virtual storage for
the SEGTABLE • 40 bytes/64K of virtual storage for
the PAGTABLE • 136 bytes/64K of virtual storage for
the SWPTABLE • 56 bytes/virtual device • 40 bytes/virtual control unit • 40 bytes/virtual channel
Figure 14. Real storage Requirements for CP Requirements (Part 2 of 2)
For example, if you have: 1M of real storage
29 real dev ices
6 real cont rol units
3 real channels
and 12 virtual machines defined, each with:
1 virtual reader
1 virtual printer
1 virtual punch
3 v irtual disks
3 virtual channels
1 virtual machine console
3 virtual control units 320K of virtual storage
you would estimate CP real storage requirements as follows.
152K for the CP resident nucleus
16K for the CP internal trace tabl-e (see "SYSCOR Macro") 4K for the real control blocks, calculated as follows: 60K 232K
88 X 29 = 2552 bytes for the real devices
72 X 6 = 432 bytes for the real control units
96 X 3 = 288 bytes for the real channels
the sum is: 2552 + 432
(approximately 4K)
+ 288 = 3272 bytes
for permanently allocated free storage (default value)
real storage required
Also, as each of the 12 virtual machines defined logs on, approximately
2K of real storage is allocated to each from the permanently allocated
free storage.
88 IBM VM/370 Planning and System Generation Guide