Home >  Resources > HACMP Resources Collection > SSA Basics

This page discusses various basic SSA concepts. It is a long way from being a complete description of the topic (please refer to the various IBM SSA manuals and redbooks for details). This page is a companion page to SSA in a High Availability Context.

The page is too short. Why don't you help fix that problem?

This page is part of the Matilda Team's HACMP Resources Collection. The home page of the collection is located here.

IMPORTANT: read the disclaimer BEFORE you use any information provided in this collection.

7133-related issues

Relationship between 7133 SSA connectors and disk drive modules

In all other 7133 models (as far as we know), the connectors are labeled as follows: Note the absence of the J's and the different arrangement and values of connector labels. In these models, the connector number corresponds directly to the disk drive module number that it connects to - connector 1 connects to disk drive module 1, connector 4 connects to disk drive module 4, etc.

Here's a table that gives the complete relationships:

Model 010 and 500 Connectors	Disk Drive Modules	All Other Model Connectors
J9 and J10	Back disk drive modules 1 through 4	1 and 4
J5 and J6	Back disk drive modules 5 through 8	5 and 8
J3 and J4	Front disk drive modules 9 through 12	9 and 12
J1 and J2	Front disk drive modules 13 through 16	13 and 16

This table is stolen nearly verbatim from IBM's 7133 SSA Disk Subsystems Service Guide SY33-0185-02.

Bypass vs Forced Inline Mode

All 7133 models with the exception of the Model 010 and Model 500 support a feature called bypass mode. Each of the pairs of SSA connectors (1/16, 4/5, 8/9, and 12/13) can be individually configured to operate in bypass mode or forced inline mode.

If configured in bypass mode and if neither of the SSA connectors in the pair is connected to a powered-on SSA adapter or device then the pair of connectors operates in bypass state - the pair of connectors operates as though they were directly cabled together.

In all other situations (i.e. when the pair are configured in forced inline mode or if either of the SSA connectors is connected to a powered-on SSA adapter or device) the pair operate in inline state - the pair of connectors behave as two independent connectors.

The 7133 models 010 and 500 don't support the bypass mode feature. For these two models, the SSA connector pairs act as though they are always configured in forced inline mode.

Configuring bypass / forced inline mode

Bypass mode and forced inline mode is configured by setting jumpers. In the most recent 7133 models, these jumpers are a customer-configurable option. In older models, customers are not supposed to touch these jumpers. If you aren't sure if your 7133 model has customer-configurable jumpers, read the installation manual that was supplied with the unit. If it explains how to set the jumpers then it is likely that they are customer-configurable (if there is any doubt, check with your local IBM service organization).

Regardless of whether or not your 7133 model's jumpers are customer configurable, it is useful to understand where these jumpers are and what they do.

IMPORTANT: it is VERY EASY to break something that will be rather expensive to fix if you try to follow these instructions and either make a mistake or discover that they are wrong - be VERY CAREFUL!

If you look carefully at the SSA connectors in a 7133 which supports the bypass feature (any 7133 except the 010 and 500 models), you'll notice that each pair of SSA connectors is on a separate metal plate. This metal plate is held onto the 7133 chassis by two screws. Remove the screws on one of the plates and carefully pull on the pair of SSA connectors. Once you overcome the initial resistance, you'll discover that you're holding the end of a fairly long, thin and somewhat fragile looking circuit board.

This assembly (i.e. the two SSA connectors, the metal plate and the long thin circuit board) is called a bypass card. This bypass card has jumpers or jumper blocks located on it which can be inserted onto the card in two ways (actually, there will be two correct ways and possibly a whole bunch of incorrect ways - be VERY careful). Moving these jumpers or jumper blocks from one (correct) position to the other (correct) position will flip the card between bypass mode and forced inline mode. Once you've got the jumpers or jumper blocks correctly configured, carefully re-install the bypass card and you're done.

We believee that bypass cards are hot swapable so you should be able to change them while the 7133 is powered on (check the installation and/or service manuals to be sure). Obviously, data access to the 7133 disks via the connectors on the bypass card is impossible while the card is removed so changing bypass jumpers is likely to involve an outage in any case (i.e. get them right the first time).

A Simple Example

The bypass mode feature comes in handy in many situations. Consider the following SSA configuration in which a single 7133 is divided into two separate 8 disk loops with each loop being connected to two different SSA adapter cards:

The 4/5 and the 12/13 SSA connector pairs are configured in bypass mode and the 1/16 and 8/9 SSA connector pairs are configured in forced inline mode. This will ensure that the loss of either adapter will result in a valid and operational configuration. For example, if the left adapter or, more likely, the system unit containing the left adapter losses power then SSA connectors 4 and 5 will switch to bypass state whereas SSA connectors 8 and 9 will remain in inline state as will SSA connectors 1 and 16. The result will be that the red loop (disk drive modules 1 through 8) will remain completely intact (i.e. no break will occur between disk drive modules 4 and 5). The blue loop (disk drive modules 9 through 16) will be broken between the 9 and 16 disk drive modules although it will remain operational since the two quads are still accessible via connectors 12 and 13.

An even simpler example

A single 7133 can be configured as a single SSA loop attached to a pair of SSA adapters. For example, consider the following configuration:

In this configuration, all four sets of bypass cards are configured in bypass mode. In addition, connectors 4, 5, 12 and 13 are not connected to anything. Since the 4/5 and the 12/13 pairs are configured in bypass mode, they'll always operate in bypass state (i.e. disk drive modules 4 and 5 will connected together as will disk drive modules 12 and 13). If either adapter card loses power, the pair of connectors connected to the failed adapter card will switch to bypass state which will maintain complete loop integrity.

If the B pair of SSA connectors on the adapters are actually available for use then an arguably better configuration would involve using the B pair of connectors on the left adapter to connect to the 4/5 pair of 7133 SSA connectors and use the B pair of connectors on the right adapter to connect to the 12/13 pair of 7133 connectors. All four pairs of 7133 connectors would remain configured in bypass mode. Of course, an even better configuration would be to just use the configuration in the previous example.

Bypass / Forced Inline Mode Summary

These are just a handful of examples of how to use the bypass mode feature. It is safe to say that every 7133 configuration will require that the bypass mode feature be correctly configured in order to ensure that the 7133 configuration works as required in all relevant failure modes.

A Few Final Comments

The 7133 model T40 (and possibly other models) also support a mode called forced open which can be used to disable pairs of connectors. In addition, the 7133 (and possibly other models) can be configured via host-based utility programs or using the jumpers described above. If configured in forced-inline mode using the jumpers then the ability to change the configuration using the host-based utility program is disabled.

Start SSA configuration planning by thinking about loops of SSA disks

When you're planning your SSA configuration, plan SSA loops that only contain quads of 7133 disk drive modules (i.e. no SSA adapters). Once you've got these loops worked out, decide where in each loop you want to put an SSA adapter. In my experience, taking this approach tends to result in cleaner SSA configurations. Of course, your experience may differ . . .

Before all else fails, read the manuals . . .

This page is intended as an overview of some of the HACMP-related SSA issues. It is NOT intended to replace the various IBM SSA publications. There are some less than obvious issues that you need to watch out for as you plan your SSA configurations. Failure to follow all of the SSA rules could result in some rather prolonged learning experiences.

IMPORTANT: If you lack the appropriate skills, experience and/or competency, are unwilling to take responsibility for your actions, or if you don't like these disclaimers then don't use this information.