path. If the RCUBLOK is marked scheduled and the present request will not
release the control unit (example: TAPE FSF and TAPE BSF), theIOBLOK is
queued off theRCUBLOK and a search is made for an alternate control unit path.
If theRCUBLOK is marked scheduled and the present request will release the con
trol unit, the search continues for a channel path. If theRCUBLOK is not marked
scheduled or busy but there are otherI/O requests queued on the RCUBLOK, the
check is again made to see if the present request will release the control unit. If the
present request will not release the control unit, the request is queued and a search
is made for an alternate control unit path. Otherwise, the search continues for a
channel path.
TheRCUBLOK "busy" and "scheduled" indicators are only turned on for shared
control units. The busy and scheduled indicators are turned on in theRCUBLOK for tape and 2314 DASD control units. The non-shared DASD RCUBLOKS never
have the busy and scheduled indicators in the"on" status. For this reason, alter
nate control unit path selection rarely takes place for non-shared control units. The
one exception occurs when the channel path through the first control unit appears
busy (because a real channel busy condition was encountered). If an alternate path
exists through a second control unit, the control blocks associated with the second
control unit path are examined.
Finding an available channel path is the final step prior to issuing theSIO. If the RCHBLOK is marked busy, a search is made for an alternate channel path. If the RCHBLOK has other requests queued on the RCHBLOK, a search for an alter
nate channel path is made.VM/370 never marks a byte multiplexor RCHBLOK busy. The only time a block multiplexor is marked busy is after a condition code 2
has been encountered. TheI/O load on block multiplexor channels must be suffi
cient to cause channel busy conditions before path selection on an alternate chan
nel can take place.
MVS/System Extensions Support
The MVS/System Extensions support in VM/SP allows an MVS system running in
a virtual machine to exploit the enhancements available in the MVS/System Exten
sions Program Product (Program No. 5740-XEl) if the System/370 Extended
Facility or System/370 Extended Feature is present on the hardware.
Included in the MVS/System Extensions Program Product enhancement is the use
of:
1. The System/370 Extended Facility for the303x and the 308x processors, or
2. The System/370 Extended Feature for the System/370 Model 158 and 168
processors, or
3. ECPS:MVS for the 4341.
Note: An RPQ (MK3272) is available for the 158-3 processor that allows the
coexistence of virtual machine assist and System/370 Extended Facility(S370E) and VM/370 Extended Feature. Thus, an MVS/SE virtual machine can run under
VM/SP with virtual machine assist active on a 158-3 processor. ECPS:MVS and
ECPS:VM/370 are mutually exclusive in the 4341 Model Group 1 and 4341 Mod
el Group 2. The control storage expansion feature of the Model Group 2 allows
coexistence of ECPS:MVS and ECPS:VM/370.
PerformanceGuidelines 45
release the control unit (example: TAPE FSF and TAPE BSF), the
queued off the
If the
trol unit, the search continues for a channel path. If the
scheduled or busy but there are other
check is again made to see if the present request will release the control unit. If the
present request will not release the control unit, the request is queued and a search
is made for an alternate control unit path. Otherwise, the search continues for a
channel path.
The
control units. The busy and scheduled indicators are turned on in the
have the busy and scheduled indicators in the
nate control unit path selection rarely takes place for non-shared control units. The
one exception occurs when the channel path through the first control unit appears
busy (because a real channel busy condition was encountered). If an alternate path
exists through a second control unit, the control blocks associated with the second
control unit path are examined.
Finding an available channel path is the final step prior to issuing the
nate channel path is made.
has been encountered. The
cient to cause channel busy conditions before path selection on an alternate chan
nel can take place.
MVS/System Extensions Support
The MVS/System Extensions support in VM/SP allows an MVS system running in
a virtual machine to exploit the enhancements available in the MVS/System Exten
sions Program Product (Program No. 5740-XEl) if the System/370 Extended
Facility or System/370 Extended Feature is present on the hardware.
Included in the MVS/System Extensions Program Product enhancement is the use
of:
1. The System/370 Extended Facility for the
2. The System/370 Extended Feature for the System/370 Model 158 and 168
processors, or
3. ECPS:MVS for the 4341.
Note: An RPQ (MK3272) is available for the 158-3 processor that allows the
coexistence of virtual machine assist and System/370 Extended Facility
VM/SP with virtual machine assist active on a 158-3 processor. ECPS:MVS and
ECPS:VM/370 are mutually exclusive in the 4341 Model Group 1 and 4341 Mod
el Group 2. The control storage expansion feature of the Model Group 2 allows
coexistence of ECPS:MVS and ECPS:VM/370.
Performance