or

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n

6.11.3 Parameter

devaddr

This is the device address.

devtype

This is the device type. Valid device types are 2311, 2314, 3330, 3350, 3375, 3380,
3390 and 9345.

filename

This specifies the filename (and optionally the path) of the file containing the DASD
image.

shadowfile

A shadow file contains all the changes made to the emulated DASD device since it
was created, maintained until the next shadow file is created. The moment of the
shadow file’s creation can be thought of a snapshot of the current emulated DASD
at that time, as if the shadow file is later removed the emulated DASD reverts back
to the state it was when the snapshot was taken.

Using shadow files, the base files can be kept on a read-only device like CDROM
or can be defined as read-only thus ensuring that these files can never be corrupt-
ted.

The shadow file does not have to actually exist when it is defined in the configu-
ration file. The shadow operand of the sf= parameter is simply a filename template
that will be used to name the shadow file whenever one is be created. Shadow files
are created using the sf+ xxxx or sf+ * commands.

The shadow file name must have a position where the shadow file number will be
set. This is either the character preceding the last period after the slash or the last
character if there is no period. For example: sf=shadows/linux1_*.dsk.

Hercules console commands are provided to add a new shadow file, remove the
current shadow file (with or without backward merge), compress the current
shadow file and display the shadow file status and statistics.

Please note, that the “sf” parameter has to be coded in lowercase letters otherwise
an error message is presented.

Details on how to work with shadow files can be found in chapter

[NO]SYNCIO

SYNCIO enables possible “synchronous” I/O. This is a DASD I/O feature wherein
guest I/O requests are completed “synchronously” during the actual emulated exe-
cution of the SIO / SSCH (START IO / START SUBCHANNEL) instructions rather
than being deferred and executed asynchronously in a separate device I/O thread.

Only I/O which are known to be able to be completed without actually needing to
perform any actual host I/O are completed synchronously (e.g. whenever the data
being requested is found already be in the cache). If the requested data could not
be found in the cache then actual host I/O will need to be done and the request is

passed to a device I/O thread to be completed asynchronously instead.

SYNCIO is the default for CKD devices. SYNCIO statistics may be displayed via
the Hercules SYNCIO console command. SYNCIO may be abbreviated as SYIO.

Note: If you plan on using SYNCIO with Linux/390 and/or zLinux you might also
want to take a look at the IODELAY system parameter as well.

READONLY

READONLY returns “write inhibited” sense when a write is attempted. Note that not
all the sense bits may be set absolutely correctly.

FAKEWRITE

FAKEWRITE is a workaround for the READONLY sense problem that was men-
tioned above. In these cases the disk is not intended to be updated (e.g. MVS up-
dates the DSCB last referenced field for a readonly file) and any writes appear to
be successful even though nothing is actually written.

type

Specifies the type of control unit to which this device is attached. The use of this
parameter does not necessarily imply that all functions of the specified control unit
are emulated. The purpose is to force a particular control unit type to be indicated
in the data returned by SENSE ID and similar CCW's.

Normally the default value is appropriate and this parameter need not to be speci-
fied. Additional to the table below the following values may be specified: 3990-3
and 3990-6. The default value depends on the device type as shown in table below.

ipname

This is the host name or IP address of the system where the Hercules shared de-
vice server is running.

port

This is the port number the shared device server is listening on. If omitted the de-
fault is 3990.

devnum

Devnum specifies the device number on the shared device server. If omitted the
default is the current device number on the local system.

COMP=n

This keyword requests that the data has to be transferred compressed between the
client and the server. The argument n specifies the compression level (1-9). Values
closer to 1 mean less compression but also less processor time to perform the
compression. Values closer to 9 mean the data is compressed more but also more
processor time is required to compress the data.

Two other options LAZYWRITE and FULLTRACKIO have been deprecated. They are still accepted to
support compatibility with older configuration files but do actually nothing. It is strongly recommended to
remove these statements from the configuration file.

The following table shows the default Control Unit types depending on on the device type:

Device Type

Default CU Type

2311

2841

2314

2314

3330, 3340, 3350, 3375, 3380

3880