39.2. iSCSI Settings With dm-multipath

If dm-multipath is implemented, it is advisable to set iSCSI timers to immediately defer commands to the multipath layer. To configure this, nest the following line under device { in /etc/multipath.conf:
features		"1 queue_if_no_path"
This ensures that I/O errors are retried and queued if all paths are failed in the dm-multipath layer.
You may need to adjust iSCSI timers further to better monitor your SAN for problems. Available iSCSI timers you can configure are NOP-Out Interval/Timeouts and replacement_timeout, which are discussed in the following sections.

39.2.1. NOP-Out Interval/Timeout

To help monitor problems the SAN, the iSCSI layer sends a NOP-Out request to each target. If a NOP-Out request times out, the iSCSI layer responds by failing any running commands and instructing the SCSI layer to requeue those commands when possible.
When dm-multipath 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 dm-multipath 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/iscsi/iscsid.conf 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[10]. To adjust this, open /etc/iscsi/iscsid.conf 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 replacement_timeout seconds. For more information about replacement_timeout, refer to Section 39.2.2, “replacement_timeout.
To verify if the SCSI Error Handler is running, run:
# iscsiadm -m session -P 3

[10] Prior to Red Hat Enterprise Linux 5.4, the default NOP-Out requests time out was 15 seconds.