Serial DASD Fence Command

Kernel and Subsystems Technical Reference, Volume 2

Serial DASD Fence Command

The Serial Direct Acess Storage Device (DASD) controller supports up to eight hosts and can fence, or lock out, specified hosts. Fences are established and removed via the Serial DASD subsystem Fence command. Once a fence is established, it can only be removed with another Fence command or by cycling the power to the controller.

Hardware Implementation

Each DASD has an associated two-byte fence register in the controller. A bit set to 1 indicates the host attached to the specified host connector on the back of the controller drawer can only issue the Inquiry, Request Sense, Fence, and Read(10) Serial DASD subsystem commands. The read with reservation (RWR) bit must be set in the command descriptor block. The host connectors on the back of the controller drawer are labeled from 0-7, and the fence register bits are ordered from left to right. The host connector bit is 0.

The following diagram illustrates the Serial DASD Fence command descriptor block:

The command descriptor block for the Fence command contains the following fields:

Operation Code

Specifies the operation code for this command. The value of this field is always D0h.

LUN

Logical unit number. The value of this field is always 000.

Force Fence

Specifies how to run the Fence command. It can have the following values:

0	Runs the Fence command from a host that is not fenced-out.
1	Runs the Fence command from a fenced-out host.

Command Modifier

Specifies which type of fencing operations is to be used. The command modifier can have one of the following values:

0001

Indicates the Mask and Swap operation performs the following equation in order to set the DASD's fence bit register:

(fence data field & fence mask field) | (old fence register value & ~fence mask field)

That is, the fence mask field specifies which bits in the fence data field should be used.

The Mask and Swap operation allows fences for multiple hosts to complement one another, since they do not need to know the current state of the fence to fence out another host. For example, if host0 and host3 are currently fenced out, then the fence register will have the value of 1001000000000000. To unfence host3 using Mask and Swap requires the fence data field to be set to 0000000000000000 and the fence mask field to be set to 0001000000000000.

0010

Indicates the Compare and Swap fencing operation sets the DASD's fence bit register, depending on the value of the fence mask field. If the old fence bit register equals fence mask field, the fence bit register is set equal to the fence data field. Otherwise, the fence bit register remains unchanged.

Fence mask

Indicates a 16-bit fence mask field. Its value is dependent on the value of the Command Modifier component.

Fence data

Indicates a 16-bit fence data field. Its value is dependent on the value of the Command Modifier component.

Reserved

The value of this field is always 0000000000000000.

Allocation Length

The value of this field is always 00000100.

VU

The value of this field is always 00.

Reserved

The value of this field is always 0000.

Flag

The value of this field is always 0.

Link

The value of this field is always 0.

The Fence command also supplies a method to determine which hosts are currently fenced out as well as which tail the current host is connected via the data returned from the Fence command. The following diagram illustrates the data returned by the Fence command.

A fence cannot be removed by a reset. It can only be removed by cycling power on the controller of the DASD, or by issuing a Fence command.

Software Implementation

The Serial DASD subsystem device driver uses the hardware's Mask and Swap fence commands to set and remove fences. The device driver also uses the Mask and Swap fence command with the fence mask set to all zeroes to determine its current host position.

The Serial DASD controller configuration method enables the fencing mechanism. Once enabled, a user can create or remove fences with ioctls to individual DASDs. The device driver maintains a fence by reestablishing it whenever the DASD is powered off and on.

Related Information

Serial DASD Subsystem Device Driver.

Serial DASD Concurrent Mode of Operation Interface.

Device-Dependent Subroutines for Serial DASD Operations.

Device-Dependent Subroutines for Serial DASD Adapter Operations.

Device-Dependent Subroutines for Serial DASD Controller Operations.

Serial Direct Access Storage Device (DASD) Overview in AIX Version 4.3 Kernel Extensions and Device Support Programming Concepts.