System information

When d m-mul ti path is being used, the SCSI layer will fail those running commands and defer them
to the multipath layer. The multipath layer then retries those commands on another path. If d m-
mul ti path is not being used, those commands are retried five times before failing altogether.
Intervals between NOP-Out requests are 10 seconds by default. To adjust this, open
/etc/i scsi /i scsi d . co nf and edit the following line:
node.conn[0].timeo.noop_out_interval = [interval value]
Once set, the iSCSI layer will send a NOP-Out request to each target every [interval value] seconds.
By default, NOP-Out requests time out in 10 seconds ⁠ . To adjust this, open
/etc/i scsi /i scsi d . co nf and edit the following line:
node.conn[0].timeo.noop_out_timeout = [timeout value]
This sets the iSCSI layer to timeout a NOP-Out request after [timeout value] seconds.
SCSI Error Handler
If the SCSI Error Handler is running, running commands on a path will not be failed immediately
when a NOP-Out request times out on that path. Instead, those commands will be failed after
repl acement_ti meo ut seconds. For more information about repl acement_ti meo ut, refer to
Section 25.16.2.2, “ repl acement_ti meo ut” .
To verify if the SCSI Error Handler is running, run:
# iscsiadm -m session -P 3
25.1 6 .2 .2 . repl acement_ti meo ut
repl acement_ti meo ut controls how long the iSCSI layer should wait for a timed-out path/session
to reestablish itself before failing any commands on it. The default repl acement_ti meo ut value is
120 seconds.
To adjust repl acement_ti meo ut, open /etc/i scsi /i scsi d . co nf and edit the following line:
node.session.timeo.replacement_timeout = [replacement_timeout]
The 1 q ueue_i f_no _path option in /etc/mul ti path. co nf sets iSCSI timers to immediately
defer commands to the multipath layer (refer to Section 25.16.2, “ iSCSI Settings With d m-
mul ti path” ). This setting prevents I/O errors from propagating to the application; because of this,
you can set repl acement_ti meo ut to 15-20 seconds.
By configuring a lower repl acement_ti meo ut, I/O is quickly sent to a new path and executed (in
the event of a NOP-Out timeout) while the iSCSI layer attempts to re-establish the failed path/session.
If all paths time out, then the multipath and device mapper layer will internally queue I/O based on the
settings in /etc/mul ti path. co nf instead of /etc/i scsi /i scsi d. co nf.
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