Document revision date: 19 July 1999

The function-dependent arguments for IO$_CREATE, IO$_ACCESS, IO$_DEACCESS, IO$_MODIFY, and IO$_DELETE are as follows:

• P1---The address of the file information block (FIB) descriptor.
• P2---The address of the file name string descriptor (optional). If specified, the name is entered in the directory specified by the FIB.
• P3---The address of the word that is to receive the length of the resulting file name string (optional).
• P4---The address of a descriptor for a buffer that is to receive the resulting file name string (optional).
• P5---The address of a list of attribute descriptors (optional). If specified, the indicated attributes are read (IO$_ACCESS) or written (IO$_CREATE, IO$_DEACCESS, and IO$_MODIFY).

See Chapter 1 for more information on these functions.

The function-dependent arguments for IO$_READVBLK, IO$_READLBLK, IO$_WRITEVBLK, and IO$_WRITELBLK are as follows:

• P1---The starting virtual address of the buffer that is to receive the data from a read operation; or, in the case of a write operation, the virtual address of the buffer that is to be written on the disk. On OpenVMS Alpha, P1 can be a 64-bit address.
• P2---The number of bytes that are to be read from the disk, or written from memory to the disk. An even number must be specified if the controller is an RK611, RL11, RX211, or UDA50.
• P3---The starting virtual/logical disk address of the data to be transferred in a read operation; or, in a write operation, the disk address of the area that is to receive the data.
  In a virtual read or write operation, the address is expressed as a block number within the file, that is, block 1 of the file is virtual block 1. (Virtual block numbers are converted to logical block numbers using mapping windows that are set up by the file system ACP process.)
  In a logical read or write operation, the address is expressed as a block number relative to the start of the disk. For example, the first sector on the disk contains block 0 (or at least the beginning of block 0).

The function-dependent arguments for IO$_WRITEVBLK, IO$_WRITELBLK, and IO$_WRITEPBLK functions that include the IO$M_ERASE function modifier are as follows:

• P1---The starting virtual address of the buffer that contains a 4-byte, user-specified erase pattern. If the P1 address is 0, a longword of 0 will be used for the erase pattern. If the P1 address is nonzero, the contents of the 4 bytes starting at that address will be used as the erase pattern. Compaq recommends that the user specify a P1 address of 0 to lower system overhead. On OpenVMS Alpha, P1 can be a 64-bit address.

  Note DSA disk controllers provide controlled, assisted erasing for the IO$M_ERASE modifier (with virtual and logical write functions) only when the erase pattern is all zeros. If a nonzero erase pattern is used, there is a significant performance degradation with these disks. DSA disks do not accept physical QIO transfers.

• P2---The number of bytes of erase pattern to write to the disk. The number specified is rounded up to the next highest block boundary (512 bytes).
• P3---The starting virtual, logical, or physical disk address of the data to be erased.

The function-dependent arguments for IO$_WRITECHECK, IO$_READPBLK, and IO$_WRITEPBLK are as follows:

• P1---The starting virtual address of the buffer that is to receive the data in a read operation; or, in a write operation, the starting virtual address of the buffer that is to be written on the disk. Passed by reference. On OpenVMS Alpha, P1 can be a 64-bit address.
• P2---The number of bytes that are to be read from the disk, or written from memory to the disk. Passed by value. An even number must be specified if the controller is an RK611, RL11, or UDA50.
• P3---The starting physical disk address of the data to be read in a read operation; or, in a write operation, the starting physical address of the disk area that is to receive the data. Passed by value. The address is expressed as sector, track, and cylinder in the format shown in Figure 2-5. (On the RX01 and RX02, the high word specifies the track number rather than the cylinder number.) Check the UCB of a currently mounted device to determine the maximum physical address value for that type of device.

  Note On the RB80 and RM80, do not address cylinders 560 and 561. These two cylinders are used for diagnostic testing only.

The function-dependent argument for IO$_SEARCH is as follows:

• P1---The physical disk address where the tape is positioned. The address is expressed as sector, track, and cylinder in the format shown in Figure 2-5.

Figure 2-5 Starting Physical Address

The function-dependent argument for IO$_SEEK is as follows:

• P1---The physical cylinder number where the disk heads are positioned. The address is expressed in the format shown in Figure 2-6.

Figure 2-6 Physical Cylinder Number Format

The function-dependent argument for IO$_FORMAT is as follows:

• P1---The density at which an RX02 diskette is reformatted (see Section 2.4.4).

The read function reads data into a specified buffer from disk starting at a specified disk address.

The operating system provides the following read function codes:

• IO$_READVBLK---Read virtual block
• IO$_READLBLK---Read logical block
• IO$_READPBLK---Read physical block

If a read virtual block function is directed to a volume that is mounted foreign, that function is converted to read logical block. If a read virtual block function is directed to a volume that is mounted structured, the volume is handled in the same way as for a file-structured device.

Three function-dependent arguments are used with these codes: P1, P2, and P3. These arguments are described in Section 2.4.

The data check function modifier (IO$M_DATACHECK) can be used with all read functions. If this modifier is specified, a data check operation is performed after the read operation completes. A data check operation is also performed if the volume that has been read, or the volume on which the file resides (virtual read) has the characteristic "data check all reads." Furthermore, a data check is performed after a virtual read if the file has the attribute "data check on read." The RX01 and RX02 drivers do not support the data check function.

If IO$M_DATACHECK is specified with a read function code to a TU58, or if the volume read has the characteristic "data check all reads," a read check operation is performed. This alters certain TU58 hardware parameters when the tape is read. (The read threshold in the data recovery circuit is increased; if the tape has any weak spots, errors are detected.)

The data check function modifier to a disk or tape can return five error codes in the I/O status block:

SS$_CTRLERR	SS$_DRVERR	SS$_MEDOFL
SS$_NONEXDRV	SS$_NORMAL

If no errors are detected, the disk or tape data is considered reliable.

The inhibit retry function modifier (IO$M_INHRETRY) can be used with all read functions. If this modifier is specified, all error recovery attempts are inhibited. IO$M_INHRETRY takes precedence over IO$M_DATACHECK. If both are specified and an error occurs, there is no attempt at error recovery and no data check operation is performed. If an error does not occur, the data check operation is performed.

2.4.2 Write

The write function writes data from a specified buffer to disk starting at a specified disk address.

The operating system provides the following write function codes:

• IO$_WRITEVBLK---Write virtual block
• IO$_WRITELBLK---Write logical block
• IO$_WRITEPBLK---Write physical block

If a write virtual block function is directed to a volume that is mounted foreign, the function is converted to write logical block. If a write virtual block function is directed to a volume that is mounted structured, the volume is handled in the same way as for a file-structured device.

Three function-dependent arguments are used with these codes: P1, P2, and P3. These arguments are described in Section 2.4.

The data check function modifier (IO$M_DATACHECK) can be used with all write operations. If this modifier is specified, a data check operation is performed after the write operation completes. A data check operation is also performed if the volume written, or the volume on which the file resides (virtual write), has the characteristic "data check all writes." Furthermore, a data check is performed after a virtual write if the file has the attribute "data check on write." The RX01 and RX02 drivers do not support the data check function.

If IO$M_DATACHECK is specified with a write function code to a TU58, or if the volume written has the characteristic "data check all writes," a write check operation is performed. The write check verifies data written on the tape. First, the specified data is written on the tape. Then the tape is reversed and the TU58 controller reads the data internally to perform a checksum verification. If the checksum verification is unsuccessful after eight attempts, the write check operation is aborted and an error status is returned.

The inhibit retry function modifier (IO$M_INHRETRY) can be used with all write functions. If that modifier is specified, all error recovery attempts are inhibited. IO$M_INHRETRY takes precedence over IO$M_DATACHECK. If both IO$M_INHRETRY and IO$M_DATACHECK are specified and an error occurs, there is no attempt at error recovery, and no data check operation is performed. If an error does not occur, the data check operation is performed. IO$M_INHRETRY has no effect on DSA disks.

The write deleted data function modifier (IO$M_DELDATA) can be used with the write physical block (IO$_WRITEPBLK) function to the RX02. If this modifier is specified, a deleted data address mark instead of the standard data address mark is written preceding the data. Otherwise, the operation of the IO$_WRITEPBLK function is the same; write data is transferred to the disk. When a successful read operation is performed on this data, the status code SS$_RDDELDATA is returned in the I/O status block rather than the usual SS$_NORMAL status code.

The IO$M_ERASE function modifier can be used with all write function codes to erase a user-selected part of a disk. This modifier propagates an erase pattern through the specified range. Section 2.4 describes the write function arguments to be used with IO$M_ERASE.

2.4.3 Sense Mode

Sense mode operations obtain current disk device-dependent characteristics that are returned to the caller in the second longword of the I/O status block (see Figure 2-8). The operating system provides the following function codes:

• IO$_SENSEMODE---Sense characteristics
• IO$_SENSECHAR---Sense characteristics

IO$_SENSEMODE is a logical function. IO$_SENSECHAR is a physical I/O function and requires the access privilege necessary to perform physical I/O. No device- or function-dependent arguments are used with either function.

2.4.4 Set Density

The set density function assigns a new density to an entire RX02 floppy diskette. The diskette is also reformatted: new data address marks are written (single or double density) and all data fields are zeroed. Set density is a physical I/O function and requires the access privilege necessary to perform physical I/O. The following function code is provided:

• IO$_FORMAT

IO$_FORMAT takes the following function-dependent argument:

• P1---The density at which the diskette is reformatted:
  • 0 = single density (default)
  • 1 = single density
  • 2 = double density

The set density operation should not be interrupted before it is completed (about 15 seconds). If the operation is interrupted, the resulting diskette might contain illegal data address marks in both densities. The diskette must then be completely reformatted and the function reissued.

2.4.5 Search

The search function positions a TU58 magnetic tape to the block specified. Search is a physical I/O function and requires the access privilege necessary to perform physical I/O. The operating system provides a single function code:

• IO$_SEARCH

This function code takes the following function-dependent argument:

• P1---Specifies the block where the read/write head will be positioned. The low byte contains the sector number in the range 0 to 127; the high byte contains the track number in the range 0 to 3.

IO$_SEARCH can save time between read and write operations. For example, nearly 30 seconds are required to completely rewind a tape. If the last read or write operation is near the end of the tape and the next operation is near the beginning of the tape, the search operation can begin after the last operation completes, and the tape will rewind while the process is otherwise occupied. (The search QIO is not completed until the search is completed. Consequently, if a $QIOW system service request is issued, the process will be held up until the search is completed.)

2.4.6 Pack Acknowledge

The pack acknowledge function sets the volume valid bit for all disk devices. Pack acknowledge is a physical I/O function and requires the access privilege to perform physical I/O. If directed to an RX02 disk, pack acknowledge also determines the diskette density and updates the device-dependent information returned by $GETDVI item codes DVI$_CYLINDERS, DVI$_TRACKS, DVI$_SECTORS, DVI$_DEVTYPE, DVI$_CLASS, and DVI$_MAXBLOCK. If directed to a DSA disk, pack acknowledge also sends the online packet to the controller. The following function code is provided:

• IO$_PACKACK

This function code takes no function-dependent arguments.

IO$_PACKACK must be the first function issued when a volume (pack, cartridge, or diskette) is placed in a disk drive. IO$_PACKACK is issued automatically when the DCL commands INITIALIZE or MOUNT are issued.

For DSA disks, the IO$_PACKACK function locks the drive's port selector on the port that initiated the pack acknowledge function.

In addition, the IO$_PACKACK function updates device-dependent information about DSA disks returned by $GETDVI.

2.4.7 Unload

The unload function clears the volume valid bit for all disk drives, makes DSA disks available, and issues an unload command to the drive (spins down the volume). The unload function reverses the function performed by pack acknowledge (see Section 2.4.6). The following function code is provided:

• IO$_UNLOAD

This function takes no function-dependent arguments.

2.4.8 Available

The available function clears the volume valid bit for all disk drives; that is, it reverses the function performed by pack acknowledge (see Section 2.4.6). No unload function is issued to the drive. Therefore, those drives capable of spinning down do not spin down. The following function code is provided:

• IO$_AVAILABLE

This function takes no function-dependent arguments.

2.4.9 Seek

The seek function directs the read/write heads to move to the cylinder specified in the P1 argument (see Sections 2.2.8 and 2.4, and Figure 2-6).

2.4.10 Write Check

The write check function verifies that data was written to disk correctly. The data to be checked is addressed using physical disk addressing (sector, track, and cylinder) (see Figure 2-5). If the request is directed to a DSA disk, you must specify a logical block number, even though IO$_WRITECHECK is a physical I/O function. The following function code is provided:

• IO$_WRITECHECK

A write QIO must be used to write data to disk before you enter this command. IO$_WRITECHECK then reads the same block of data and compares it with the data in the specified buffer. Three function-dependent arguments are used with this code: P1, P2, and P3. These arguments are described in Section 2.4.

IO$_WRITECHECK is similar to the IO$M_DATACHECK function modifier for write QIOs, except that IO$_WRITECHECK does not write the data to disk; it is specified after data is written by a separate write QIO. Nonprivileged processes can use the IO$M_DATACHECK modifier with IO$_WRITEVBLK (which does not require access privilege) to determine whether data is written correctly. The RX01 and RX02 drivers do not support the write check function.

The write check function and the data check function modifier to a TU58 can return six error codes in the I/O status block: SS$_NORMAL, SS$_CTRLERR, SS$_DRVERR, SS$_MEDOFL, SS$_NONEXDRV, and SS$_WRTLCK.

2.4.11 Set Preferred Path

The set preferred path function specifies a preferred path for DSA disks. This includes RA-series disks and disks accessed through the MSCP server. If a preferred path is specified for a disk, the MSCP disk class drivers (DUDRIVER and DSDRIVER) use the path as their first attempt to locate the disk and bring it on line as a result of a DCL command MOUNT or failover of an already mounted disk. In addition, you can initiate failover of a mounted disk in order to force the disk to the preferred path, or to use load-balancing information for disks accessed through MSCP servers.

The function code is:

• IO$_SETPRFPATH

The following is the function modifier:

• IO$M_FORCEPATH---Causes the disk class driver to select the server path with the highest load available rating.

The P1 parameter contains the address of a counted ASCII string (.ASCIC). This string is the node name of the HSC or system that is the preferred path. The node name must match an existing node that is known to the local node and if the node is a VAX or Alpha system, it must be running the MSCP server. This function does not move the disk to the preferred path.

The PHYS_IO privilege is required for IO$_SETPRFPATH and IO$M_FORCEPATH.

The following example shows the use of IO$_SETPRFPATH:

$assigndef $qiodef $iodef $exitdef dev: .ascid /$254$DUA48:/ chnl: .word 0 node: .ascic /HSC001/ .entry start,0 $assign_s devnam=dev,- chan=chnl blbc r0,done $qiow_s chan=chnl,- func=#IO$_SETPRFPATH,- p1=node done: $exit_s r0 .end start

This updates the local node I/O database to indicate that node HSC001 is the preferred path for DUA48.

2.4.11.1 Forcing a Path Change

You can move a disk that is already mounted to its preferred path by specifying the IO$M_FORCEPATH modifier. If a preferred path has not been specified for a disk that is accessed through the MSCP server, the IO$M_FORCEPATH function causes the disk class driver to use load-balancing information to select the server path with the highest-load-available rating.

IO$M_FORCEPATH does not accept any arguments. If you intend to move a disk to its preferred path, you must specify the preferred path in a separate $QIO function.

The following example shows use of the IO$M_FORCEPATH function modifier:

$assigndef $qiodef $iodef $exitdef dev: .ascid /$254$DUA197:/ chnl: .word 0 .entry start,0 $assign_s devnam=dev,- chan=chnl blbc r0,done $qiow_s chan=chnl,- func=#<IO$_SETPRFPATH!IO$M_FORCEPATH> done: $exit_s r0 .end start

Note that forcing a path change places the disk in mount verification. New I/O requests are suspended until mount verification is complete.

2.4.11.2 Using IO$_SETPRFPATH with Disks Dual-Pathed Between HSCs

You can use the IO$_SETPRFPATH and IO$M_FORCEPATH functions to load balance disks that are dual-pathed between HSCs. The IO$M_FORCEPATH function initiates failover of the disk on all nodes that have it mounted and that have a direct path to the HSCs. Since the node that issues the IO$M_FORCEPATH might not be the first one to attempt failover of the disk, it is essential that all nodes with direct connections to the HSCs specify the same preferred path for the disk. Only one node should issue the IO$M_FORCEPATH request.

2.4.11.3 Using IO$_SETPRFPATH with Disks Dual-Pathed Between Systems

You can use IO$M_FORCEPATH to load balance RA-series disks that are dual-pathed between systems running the MSCP server. Both serving nodes should specify the same preferred path. In order to move the disk between systems, the system that currently has the disk on line through its local controller should issue the IO$M_FORCEPATH request. The disk must be mounted on both serving nodes.

2.4.11.4 Using IO$_SETPRFPATH with Disks Accessed Through MSCP Servers

You can specify a preferred path for disks that are accessed through MSCP servers. However, this specification overrides any load-balancing decisions.

Note that if a disk can be accessed through both HSC and MSCP servers, you need not specify the HSC as a preferred path. HSC paths are always preferred to server paths.

Using IO$M_FORCEPATH without a preferred path causes the disk class driver to move the disk to the server with the highest available capacity.

2.4.11.5 Using IO$_SETPRFPATH with Phase I Volume Shadowing

You can specify IO$_SETPRFPATH for shadow set members, but not for virtual units. IO$M_FORCEPATH is not supported for shadow set members or virtual units.

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