NAME
max_fcp_reqs — maximum number of concurrent fibre channel requests outstanding on a Tachyon host bus adapter
VALUES
Recommended values
0-1024
While any value in the specified range is acceptable, the value specified will be rounded up to
the next higher power of 2 to determine the final value.
DESCRIPTION
The tunable
max_fcp_reqs
is used to define the maximum number of fibre channel requests that may be
outstanding on any Tachyon-based host bus adapter at any one time.
Fibre channel requests are the equivalent to I/O operations.
During startup of HP-UX, internal tables are constructed based on the value provided.
The larger the value, the more memory that will be consumed for the tables.
In all but very extreme situations, the default value indicated is sufficient
for construction of the internal tables.
If at any time more fibre channel requests are available than can be put on
the fibre, then the remaining requests must wait until a table entry frees up.
Table entries are freed as outstanding requests are completed.
Who Is Expected to Change This Tunable?
Anyone.
Restrictions on Changing
Changes to this tunable take effect at the next reboot.
When Should the Value of This Tunable Be Raised?
If the internal tables should ever fill, this may indicate that the tunable
value should be raised.
The message
SEST depleted - performance may be degraded.
will be output to the system diagnostic log if a situation ever arises where
any fibre channel request must wait for entries to become available in the
internal tables.
A check is made every hour to determine if the internal tables have filled
within the previous hour.
If so, the message is printed.
A single occurrence of the message, or a few occurrences over weeks should
not be interpreted as an explicit request to increase the value.
The occurrence of the message 2 or more times a day would be an indication
to increase the value.
What Are the Side Effects of Raising the Value of This Tunable?
Raising the value of this tunable will increase the amount of memory used
by the HP-UX kernel to construct internal tables.
Additionally, raising the value will mean that more fibre channel requests
can be outstanding on any given host bus adapter at any time.
The implication here is that the number of completions may also increase.
This could bog down the system SPU processing the completion interrupts,
or if internal host bus adapter queues were to fill, completions could be
lost and the original requests would need to be retried.
Data will NOT be lost, the operation transferring the data will be
retried.
When Should the Value of This Tunable Be Lowered?
The tunable should be lowered only on systems where memory resources are
tight (memory allocations are not always successful).
What Are the Side Effects of Lowering the Value of This Tunable?
The side effects of lowering the value of this tunable would be that some
fibre channel requests would wait until internal table entries became
available before being able to proceed.
For these cases, performance would be adversely affected.
If lowering the value of the tunable results in fibre channel requests
waiting for internal resources to become available, the following message
would appear in the diagnostic log:
SEST depleted - performance may be degraded.
What Other Tunables Should Be Changed at the Same Time?
None.
WARNINGS
All HP-UX kernel tunable parameters are release specific.
This parameter may be removed or have its meaning changed in
future releases of HP-UX.
AUTHOR
max_fcp_reqs
was developed by HP.
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