If DASD space is not a limiting factor, use double spooling. If
possible, generate a stripped-down verS10n of the virtual machine's
sFooling subsystem, eliminating those functions not used by that virtual
machine. the I/O buffer sizes as large as possible to cut down on SIO instructions.
If an installation has only enough DASD spooling space for one
sFooling subsystem and if only one virtual machine generates significant
aaounts of spooled output, then let that virtual machine do the
spooling. However, if many virtual machines spool data and must use a
co.mon pool of unit record devices, then an installation should probably
let VM/370 do the spooling. Output spool files are not scheduled for the real printer or punch
devices until one of these actions occur: • The user logs off, or VM/370 forces the user off. • The user loads an operating system via the CF 1Ft command. • The user (either manually or through an EXEC procedure) issues the CP CLOSE command to close the spool file. • VS1 with handshaking uses the diagnose interface to issue the CP CLOSE command after each job completes. • The installation modifies the operating system by adding DIAGNOSE instructions to communicate with VM/370 to close the spool files .• Thus, until one of the preceding actions occur, the spool files are
not sent to the real printer or punch. To keep spool files from
building up excessively on the spooling DASD, the user should close
these spool files periodically (such as at the end of each job). PROCESSOR MODEL AND CHANNEL MODEL DEPENDENICIES Channel checks (that is, channel data checks, channel control checks,
and interface control checks) no longer cause the virtual machine to be
reset. Thus, an operating system that now runs in a virtual machine can attempt either to recover from a channel check or to terminate in an
orderly manner.
If channel error recovery procedures in an operating system depend on
the processor model and channel model, then these tva requirements must be met for channel error recovery procedures to function reliably after
a channel check:
1. Depending upon the recovery procedures of the specific operating system running in the virtual machine, an installation may have to
generate the operating system for the same processor model on which VM/370 is to run.
2. The virtual machine configuration must have each virtual channel
corresFond to a single type and model of real channel. Section 1. General considerations 13

The second requirement means that all virtual devices on a virtual
channel must correspond to real devices on real channels; the real
channels must be identical to each other in type and model. For
example: Assume that a virtual machine has a 3330 disk on virtual
address 280 and a 3340 disk on virtual address 290 that correspond to
similar real devices on real addresses 380 and 590, respectively.
Because Doth virtual devices (280 and 290) are on a single virtual
channel (channel 2), the corresponding real devices (380 and 590) must both be on real channels that have an identical channel type and model.
By meeting this requirement, when an operating system issues a STltC (store channel ID) instruction to virtual channel 2, V8/370 can simulate
it the same way and return consistent results to the operating system.
Not only should the real channels be identical, but generally
speaking, they should be of the same type as the virtual channel. (The
virtual channel type is defined either in the OPTION statement in the
virtual machine's directory entry or by the class G CF DEFINE CHANNEL command.) Two exceptions to this general rule are: • When the real channel is a block multiplexer channel, the virtual
channel can be a selector channel. In this case, the simulated STltC instruction returns this information to the operating system: (1)
the model number of the block multiplexer channel, and (2) a channel type field indicating that the channel is operating in selector mode. • When the real channels are selector channels, the virtual channel can
be a block multiplexer channel. This specification may improve
performance when the virtual channel has devices on several real
selector channels. It allows the virtual machine to overlap channel
operations on the virtual channel and to take full advantage of the
several selector channels. However, when V8/370 simulates the STltC instruction issued to the virtual block multiplexer channel, it gives
the operating system the channel type and model number of a selector
channel, not of a block multiplexer channel. While this result is
inconsistent with the channel's operation as a block multiplexer, the
operating system should not detect, or be affected by, this
inconsistency.
For further restrictions about channel model-dependent functions,
refer to SPECIFYING VIRTUAL 8ACHINE CONSOLES To specify more than one console for a virtual machine, the virtual machine user must tell V8/370 about the existence of these additional
consoles. Operating systems may support either: • -- where different classes of system messages can
be routed to different consoles.
--or-- • Alternate consoles where the user can switch to a backup console wben-the-prIiary-console becomes inoperative. 14 IBM Operating Systems in a Virtual 5achine