Installing AP

Alternate Pathing (AP) is provided as a separate package for your Sun server system. To utilize its features you must install the following AP packages:

• SUNWapr - AP subsystem (root)
• SUNWapssp - AP subsystem (SSP)
• SUNWapu - AP subsystem (usr)
• SUNWapdoc - AP documentation

The Purpose of Alternate Pathing

AP supports Dynamic Reconfiguration (DR) and provides your Sun server system with a degree of fault recovery, defined here as the ability of a system to recover from a single-point failure in a manner that enables an uninterrupted execution of the workload. Specifically, AP provides fault recovery associated with disk and network adapter failure. If a failure occurs in

a disk or network adapter, and if an alternate adapter has been designated and is available, AP allows uninterrupted access to the disk or network. The following figure illustrates this feature:

Figure 1-1

In addition to providing fault tolerance for I/O adapters, AP supports the Dynamic Reconfiguration (DR) Detach operation. If a system module is removed via DR Detach, AP allows continued access to the disks and networks

connected via that system module as long as alternate adapters have been designated and are available on other system module(s). The following figure illustrates this feature:

Figure 1-2

Basic Alternate Pathing Concepts

This section covers basic AP concepts, and introduces the terminology that is used throughout this chapter.

Physical Path

For the purposes of AP, an I/O device is either a disk or network. The only type of disk device currently supported by AP is a SPARCStorage^TM Array (SSA). In this chapter, the term disk always refers to an SSA. An I/O adapter is the controller for an I/O device, such as an SSA adapter. A device node is a path in the devices directory that is used to specify a physical device, for example /dev/dsk/c0t0d1s0.

The term physical path refers to the electrical path from the host to a disk or network:

Figure 1-3

Meta-Disk

A meta-disk, as illustrated in Figure 1-4, is a construct that allows you to access a disk via either of two physical paths without having to reference either path explicitly within your scripts and programs. You reference a meta-disk (in your scripts and programs) using an AP-specific device node such as /dev/ap/dsk/mc0t1d1s0. (See "Device Nodes for Meta-Disks" on page 4-1.)

In the following figure the /dev/ap/dsk/mc0t1d1s0 device node is used to access a slice on a meta-disk, regardless of which pln port (pln2 or pln9) is currently handling I/O for the meta-disk.

Figure 1-4

Meta-Network

A meta-network, as illustrated in Figure 1-5, is a construct that allows you to access a network via either of two physical paths without having to reference either path explicitly within your scripts and programs. You reference a meta-network (in your scripts and programs) using a meta-network interface name such as mle1. (See "Meta-Network Interfaces" on page 6-1.)

In the following figure, mle1 is used to access a meta-network, regardless of which adapter (le1 or le6) is currently processing I/O for the meta-network.

Figure 1-5

Disk Pathgroup

A disk pathgroup, as illustrated in Figure 1-6, consists of two physical paths leading to the same SSA. When a physical path is part of a pathgroup, it is called an alternate path. An alternate path to a disk can be uniquely identified by the pln port that the alternate path uses. Only one alternate path at a time is allowed to handle disk I/O. The alternate path that is currently handling I/O is called the active alternate.

Note that, whereas a meta-disk provides a means for you to access a disk (in your scripts and programs), a disk pathgroup provides a means for you to manipulate the path to that disk (when you run AP commands). For example, to perform a switch operation (that is, change the active alternate from one alternate path to another), you reference a disk pathgroup within an apconfig(1M) command.

One of the alternate paths is designated as the primary path. The primary path is initially the active alternate. Although the active alternate changes when you perform a switch operation, the primary path remains constant. You reference a disk pathgroup by specifying the pln port (e.g., pln1) that corresponds to the primary path. (For information about determining the pln port name, see "Creating Disk Pathgroups and Meta-Disks" on page 4-2.)

For example, the following figure shows the results of using the apconfig(1M) command to switch the active alternate of a disk pathgroup:

Figure 1-6

Network Pathgroup

A network pathgroup, as illustrated in Figure 1-7, consists of two network adapters connected to the same physical network. The terms alternate path, active alternate, primary path, and switch have basically the same meaning as they do for disk pathgroups.

To specify a network pathgroup, reference the corresponding meta-network interface name, for example mle1. (Meta-network interface names are described in "Meta-Network Interfaces" on page 6-1.) For example, the following figure shows the results of using the apconfig(1M) command to switch the active alternate of a network pathgroup:

Figure 1-7

Alternate Pathing Terminology

The following terms are used in this chapter:

active alternate

The alternate path that is currently handling I/O for a pathgroup.

AP database (or simply "database")

A database that is maintained by the AP subsystem. The AP database contains all of the information needed to maintain the configuration's alternate paths.

alternate path

One of the physical paths within a pathgroup.

committed database entry

An entry in the AP database that is currently being used by AP to manage access to a disk or network. (Compare with uncommitted database entry.)

pathgroup

A set of two alternate paths that provide access to the same device or set of devices.

meta-disk

A disk abstraction that provides access to an underlying group of two physical paths to a disk.

meta-network

A network abstraction that provides access to an underlying group of two physical paths to a network.

physical path

The electrical path from the host to a disk or network.

pln port An optical link connection (OLC) module on an SSA adapter that can be connected to an SSA port.

primary path

The alternate path in a pathgroup that is initially the active alternate. The name of the primary path is used when constructing the name of the meta-disk or meta-network. The primary path does not change when a switch occurs.

SSA..A SPARCStorage Array. An SSA is a collection of disks within a hardware peripheral. The SSA provides access to each of its housed disks via two ports.

SSA port An optical link connection (OLC) module on an SSA that can be connected to a pln port.

SSA adapter

An adapter that resides on the host system and has one or two pln ports (a.k.a., SOC adapter).

switch The act of establishing a new alternate path as the path to be used for a given pathgroup. Note that switching does not change the primary path.

uncommitted database entry

An entry in the AP database that has not been committed and is therefore not currently in effect. If a pathgroup has been created but the database entry has not been committed, that pathgroup is not currently used by AP to manage access to a disk or network. If a previously committed pathgroup has been deleted, but that database entry has not been committed, that pathgroup is still being used by AP to manage access to the disk or network.

Supported Devices

AP supports SPARCStorage Arrays attached to pln ports on SSA adapters. For a list of the network devices supported by AP, see the release notes for this product.

After you set up Alternate Pathing for disks, you can use Online:DiskSuite version 3 (but not Solstice DiskSuite version 4) and Sun Volume Manager normally. However, if any of the physical disks have alternate adapters, those disks must be known to Online:DiskSuite^TM or Volume Manager by their AP meta-disk names. This requirement enables the AP system to switch to alternate paths for these disks without interfering with Online:DiskSuite or Volume Manager.

You can place your Sun server's host boot disk and the primary network interface under AP control. AP makes it possible for the system to boot unattended even if the primary network or boot disk controller is not accessible, as long as a usable alternate path for these devices is defined.

Example AP Configurations

The following diagram shows how you can use AP to provide fault tolerance for an Ethernet network and a SPARCStorage Array (SSA):

Figure 1-8

In this example, two network adapters--one each on Board 1 and Board 2--are connected to the same network. Similarly, two SSA adapters on the two boards are connected to the same SSA. In this situation, if Board 1 is to be removed via a Dynamic Reconfiguration (DR) Detach operation, AP can switch usage to Board 2 without interfering with any I/O operations that may be in progress.

AP is similar to, but not the same as, disk mirroring. Disk mirroring replicates data to separate devices and thereby achieves data redundancy. AP, on the other hand, achieves pathing redundancy. Disk mirroring and AP are complementary; you can use them together to achieve both data redundancy and pathing redundancy.

The following figure shows how AP might be used in conjunction with disk mirroring:

Figure 1-9

In this example, the mirroring occurs on top of AP, which enables switching of the underlying adapters used to implement the SSA mirror from one board to another without disruption of the disk mirroring or any active I/O.

AP and RSAS

Redundant Single Attach Station (RSAS) is a method of configuring a FDDI logical network interface with two independent pairs of Physical Layer Protocol (PHY) and Medial Access Control (MAC) entities. RSAS is a feature of FDDI 2.0. FDDI 3.0 does not support RSAS, nor does AP.

Despite the presence of multiple PHY/MAC pairs, an RSAS cannot connect directly to the counter-rotating ring, but instead must be connected via a concentrator. The advantage of RSAS over SAS is that RSAS provides

automatic failover (switching on failure). In the event of a detected error, the active interface is automatically switched to the alternate interface, ensuring continuous service.

AP meta-networks operate in a similar fashion. AP does not currently support automatic hardware failover; all switching is done manually. You can execute apconfig(1M) with its -P option at any time to execute a switch. The advantage of using an AP meta-network is that you can explicitly choose the hardware that is used to support a logical interface, and thereby prepare the system for Dynamic Reconfiguration (DR).

AP and Domains

AP cannot be used across two Dynamic System Domains. For example, suppose a board contains an adapter that is part of a pathgroup, and you move that board into a different domain. (You can only do this if the alternate path on that board is not currently active.) In this case, you can no longer switch to the alternate path on that board.