IBM Midrange System DS4000 Series Hardware Manual page 428

The same information is available in the Storage Manager Subsystem Management window
by selecting Advanced  Troubleshooting  Drive Channels, as shown in Figure 7-59.
Figure 7-59 Drive Channels window
Clicking the Show Details button provides the cumulative error statistics for the selected
channel. There is also an option to toggle the drive channel state between Optimal and
Degraded. This action is not necessary, as the channel automatically returns to an Optimal
state when the problem device is identified and excluded.
The Clear Error Counters option can be used to reset the counters. A sampling time of around
24 hours is normally sufficient to show any errors on a degraded drive-side loop. In some
cases, the pattern of errors will begin to emerge within minutes of resetting the counters. If the
counters have not been reset for an extended period, then the values might be historic rather
than reflecting the current status.
Major Event Log (MEL)
The MEL is always likely to hold some clues to drive-side problems. Quite often, we see
events logged against a faulty disk well before the error threshold is exceeded to mark it as
failed. If there is a pattern of repeated errors against a single device, then it is worth removing
it as a first step in diagnosing a degraded channel problem. It is also worth checking when the
errors first started, as this might point to some other activity that was in progress at the time.
Read Link Status Diagnostics
The Read Link Status error counts refer to link errors that have been detected in the traffic
flow of a Fibre Channel drive-side loop. The Read Link Status Diagnostics dialog retrieves the
error counts and shows the controllers, disk drives, ESMs, and Fibre Channel ports in
channel order.
By analyzing the error counts retrieved, you can determine the components on a drive-side
channel that might be experiencing problems communicating with the other storage
subsystems on the same channel. A high error count for a particular component might
indicate that it is experiencing problems.
To run Read Link Status Diagnostics, select Advanced  Troubleshooting  Run
Diagnostics  Read Link Status. The Read Link Status Diagnostics window will appear, as
shown in Figure 7-60 on page 411.
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IBM Midrange System Storage Hardware Guide