Adjusting the DMA bus memory length for the SSA adapter


How and when should the DMA bus memory length (dbmw) attribute
for the SSA adapter be adjusted?


The dbmw attribute sets the total bus address space which is
available for DMA transfers for commands which are active or
queued on SSA devices accessed via this adapter.

The default value (0x202000, or about 2MB) is sufficient for
normal circumstances.  The attribute should be changed if large
numbers of devices supporting long queues need to be kept active
with I/O.

If the system attempts to initiate more concurrent I/O requests
than can be serviced with the available DMA space, then some I/O
requests will be queued in the adapter device driver.  If this
happens, the total system performance may be affected, depending
on the workload.

Increasing this value may result in increased system performance
if the current value is causing I/O requests to be queued in the
adapter device driver.  Increasing this value may also result in 
some adapter cards not being configured if the total amount of
bus memory space requested by all of the adapter cards on a 
particular bus is greater than that available on the bus.

The maximum value allowed for the dbmw attribute is 0x902000,
which is the total amount of DMA space allocated to SSA adapters.
The amount used by each adapter can be increased, but the total
used by all SSA adapters cannot exceed 0x902000.  This illustrates
a trade-off between the number of SSA adapters and the amount of
DMA memory used by each.  If the system is I/O bottlenecked 
because of outstanding commands to the adapters, you can either
increase the DMA space for each adapter or add more adapters.
For the same amount of I/O, the results should be similar for
each scenario.

If the system is experiencing an I/O bottleneck (see "Diagnosing
Performance Bottlenecks for AIX v 3.2 and 4.1, IBM-4FAX document
number 2445), the dbmw attribute can be changed through smit 
using the "Change/Show Attributes of an SSA Adapter" screen or
using the chdev command.  The minimum increment is 0x1000, or
one 4K page frame.  If the DMA space is available, try doubling
the dbmw value and monitor its effect on the system's performance.

Support Line: Adjusting the DMA bus memory length for the SSA adapter ITEM: DB0604L
Dated: August 1997 Category: N/A
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