IBM Advanced SerialRAID Adapters SA33-3285-02, User Manual

Page: 312 / 531

between the adapters fails. The tmssa device driver automatically switches to adapters
ssa1 and ssa3 or adapters ssa1 and ssa4. The connections between nodes can be
modified while they are in use, and the target-mode interface tries to recover.
The TMSSA uses either of two methods to read and write data:
v
The blocking method, which waits until the I/O is complete or an error occurs before
it returns control to you.
v
The nonblocking method, which returns control to you immediately. With this method,
the write operation occurs at a later time. The read, operation returns the amount of
data that is available at the time of the operation. The amount of returned data is not
necessarily the same as the amount that you requested.
The tmssa device driver provides support for multiple concurrent read and write
operations for different devices. It does not provide support for multiple read or write
operations on the same device. The device driver blocks the operation until the device
is free. Read and write operations can run concurrently on a particular device.
If a working path exists between two nodes, communication works. The path must be
stable long enough for the driver to transmit the data. The maximum time taken to fail a
write operation is (A * R * T), where A is the number of adapters in the using system, R
is the number of retries as defined by TM_MAXRETRY in the
/usr/include/sys/tmscsi.h file, and T is the retry time-out period. The minimum time
taken to fail a write operation is the write time-out period. You can adjust the write
time-out period and the retry time-out period; see “TMCHGIMPARM (Change
Parameters) tmssa Device Driver ioctl Operation” on page 308.
You can use the select and poll routines to check for read and write capability, and can
also be notified of the possibility of a read or write operation.
The amount of data that can be sent by one write operation in blocking mode has no
limit, but the driver and adapter interface has been optimized for transfers of 512 bytes
or less. In nonblocking mode, enough buffer space must be available for the write
operation.
Each separate write operation is treated separately by the target, so, when reading,
each separate write operation requires a separate read operation.
Configuring the SSA Target Mode
Each using system requires its own unique node number. The SSA adapter software
specifies this node number, which is used by Target Mode SSA. The configuration
database contains the ssar device. The node_number attribute sets the number for the
node. Failure to have unique node numbers in the SSA loops causes unpredictable
results with the target-mode interface. Node numbers that are not unique cause error
logs. You can use the ssavfynn command to check for duplicate node numbers.
When the node is configured, it automatically inspects the existing SSA loops. It detects
all nodes that are using the target mode SSA interface now. Each detected node is then
added to the configuration database, if it is not already part of it. For each node that is
added, tmssaXX is created, where XX is the node number of the detected node.
292
User’s Guide and Maintenance Information