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Applies to:

Goal

Solution

Definition and Purpose:

A split expander, where the associated slot boards for a given expander
are assigned to different domains, is a valid, supported configuration
option in a Sun Fire 12K/15K/E20K/E25K. However, there are some behaviors
of split expander that should be considered prior to implementing a split
configuration. The information below is not intended to be read as
"don't use split expander". But, for some customers, the limitations
imposed on the system may dissuade its use.

1. Performance

Memory accesses through a split expander take an additional two clock
cycles (13 ns), increasing overall latency of memory operations.
If all 18 expanders in a 15K/E25K are split, memory latency to other
board sets increases ~6%.

2. Single Point Of Failure

By the very nature of sharing a component between two domains, that
component becomes a single point of failure for the two domains. The
failure of a split expander will interrupt both domains it serves.
Translating to Mean Time Between Failures (MTBF), if all 18 expanders
in a 15K/E25K are split, the MTBF is decreased (made worse) by ~5%.

3. Residual RStops

Consider the following configuration in which EX1 is split between
domains A and B:
+-----+-----+-----+
|     |     |     |
| SB0 | SB1 | SB2 |
| (A) | (A) | (B) |
|     |     |     |
+-----+-----+-----+
| IO0 | IO1 | IO2 |
| (A) | (B) | (B) |
|     |     |     |
+-----+-----+-----+
EX0   EX1   EX2

Now, suppose that a stop condition occurs in SB1. The stop can be
either a Dstop or an Rstop. The stop condition requires that the
history recording of the ASICs serving Domain A freeze recording
until a hardware state dump can be taken. So history recording must
be frozen on both EX0 and EX1 because both these expanders serve the
domain with the stop condition.

While the expander ASICs can service individual slots with complete
separation, there is only a single set of history registers per ASIC.
These history registers track transactions through the ASIC regardless
of slot. In this example, the end result is that the stop condition in
Domain A requires a history freeze for EX1. When history is frozen in
EX1 this effectively freezes history recording in Domain B...resulting
in an Rstop condition on Domain B. SMS will gather a hardware state
dump for both Domain A and Domain B, but the dump for Domain B will,
in all likelihood, be error free and uninteresting for diagnosis.

A residual Rstop is only created when the source of a stop condition
is in a split expander.

4. Delays in hpost Executions

Whenever hpost executes against a set of domain resources, it requires
exclusive access to the expander(s) for that domain. To achieve
exclusivity, hpost places a lock on the expander(s) at the start of
an hpost run. If another hpost process wants to access a locked
expander, it must wait.

Consider the example above again. Suppose a high level POST is running
on Domain A. Any attempt to run hpost for Domain B must wait until the
POST of A is complete because the POST for Domain A holds the lock on
EX1. For larger domains, the wait time can be significant. And, this
applies to any invocation of hpost including:

o setkeyswitch operations
o Collection of hardware state dumps (Dstop/Rstop)
o Domain reboots
o Domain recovery actions

5. Centerplane Bus Degradation

In a split expander configuration, the ASICs on the expanders handle
domain isolation (via the AXQ and SDI Domain Mask Registers). But from
the centerplane's point of view, it exchanges address and data in
formation with expanders, not slot boards. This introduces the concept
of a set of communicating expanders, or SOCX. Refer to the setbus man
page for more details.

In the example above, the expander ASICs ensure that, for example,
Domain A's SB1 cannot transmit information to Domain B's IO2. But,
within the centerplane, EX1 is (must be) able to communicate with EX2.
Likewise, EX1 is (must be) able to communicate with EX0. Because EX1
is split, the SOCX is EX0, EX1 and EX2.

The reason is because of the address, data, and response busses on the
centerplane. An expander must agree with other expanders it can
communicate with on which busses are available for transmission.
Suppose the address bus for EX0 was degraded to only use the low half
of the centerplane. Since EX0 can communicate with EX1, EX1 must also
only use the low address bus. Then, since EX1 is impacted, EX2 must
also be degraded. The entire SOCX is impacted if any of its member
expanders is degraded.

So, using split expanders increases the SOCX, and thereby decreases
the granularity of bus degradation. This is evident when trying to
use the setbus command:

% setbus -c cs0 -b a EX0

The expander in slot 1 communicates with slots not already listed,
and will be added to the list of boards to reconfigure.
Are you sure you want to continue the reconfiguration  (yes/no)

           Finally, degradation can cascade if a configuration is "staggered" 
           across multiple split expanders. Consider the following configuration,
           adding Domain C:

                        +-----+-----+-----+-----+-----+
                        |     |     |     |     |     |
                        | SB0 | SB1 | SB2 | SB3 | SB4 |
                        | (A) | (A) | (B) | (B) | (C) |
                        |     |     |     |     |     |
                        +-----+-----+-----+-----+-----+
                        | IO0 | IO1 | IO2 | IO3 | IO4 |
                        | (A) | (B) | (B) | (C) | (C) |
                        |     |     |     |     |     |
                        +-----+-----+-----+-----+-----+
                          EX0   EX1   EX2   EX3   EX4

           Degrading the bus configuration of any of these expanders effects all.
           The SOCX is all five expanders.

6. MaxCPU Configuration

In order to allow for MaxCPU boards to be used in a Split Expander 
configuration on a Sun Fire[TM] 12K/15K platform, you need the
Expander AXQ revision 6.3 in addition to the appropriate revision of
the SMS HPOST patch, go to patch ID 114608-09 or higher for SMS 1.3 and Patch ID 117371-02 or higher for SMS 1.4.1.
This HPOST fix will be integrated into SMS 1.5 and SMS 1.6.

7. USIV+ Configuration

In rare occurrences utilizing USIV+ 1.8 or 1.95 GHz System boards in split expander configurations are at risk for domains to reset with Dstop 
and reboot unexpectedly.  This issue has only been seen on systems running SMS 1.6 and either a 1.8 or 1.95 GHz system board in a split configuration.
This issue an be avoided by not using these system boards in a split configuration.  

        PTS provides a script to locate split expanders either on a live system

         or against Explorer output. The script is available at the following URL:



            12K/15K/E20K/E25K Domain Mapper



         When examining a state dump, the SplitSlotEnbl bit in an expander's

         master SDI can be examined to determine if the expander is split or

         not. For example:



            redxl> shsdi -v 0

            Note: Data is displayed from the currently loaded dump file.

            SDI EX00/S0    Component ID = 54317049

                    Master_Reset_Config[31:0] = 00000060

                       0   SDI_diserrlog          MResC[0]       => SDI Intern Reset

                       0   Slot0_diserrlog        MResC[1]       

                       0   Slot1_diserrlog        MResC[2]       

                    0x00   ExpID[4:0]             MResC[28:24]   

                       0   Mode[2:0]              MResC[31:29]   Master (0)

                    Master_Stop_Config[31:0]  = 01001113

                       1   DstopEnbl              MStopC[0]      

                       1   RstopEnbl              MStopC[1]      

                       0   SCIntEnbl              MStopC[2]      



                       0   L1Err->ErrPause        MStopC[3]      

                       1   Dstop->ErrPause        MStopC[4]      

                       0   L1Ecc->ScInt[1:0]      MStopC[6:5]    

                       2   L1Ecc->Rstop[1:0]      MStopC[8:7]    

                       0   L1Err->ScInt[1:0]      MStopC[10:9]   L1Slot asserted err

                       2   L1Err->Dstop[1:0]      MStopC[12:11]  L1Slot asserted err

                       0   SBBCErr->SCInt         MStopC[13]     

                       0   SBBCErr->Dstop         MStopC[14]     

                       1   EnblStopReqChk         MStopC[24]     

                       0   L1Dstop->ExpDStop      MStopC[28]     

                       0   AnyDstop->ExpDStop     MStopC[29]     

                       0   Dstop->DReset          MStopC[30]     For split exp

                       0   ShiftErrPausePhase     MStopC[31]     

                    Core_Config[21:0]   = 0D9142

                       0   Pass4TargIDDisbl       CoreC[0]       Rev 4+

                       1   Slot1=SerDom1          CoreC[1]       Rev 4+

                -->    0   SplitSlotEnbl          CoreC[5]       In master SDI (0)

            ....



         SplitSlotEnbl = 1 indicates the expander is configured for split slot.

 

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