Iscsi Root - Red Hat ENTERPRISE LINUX 5.5 - ONLINE STORAGE GUIDE Manual

Online Storage Guide
To verify if the SCSI Error Handler is running, run:
iscsiadm -m session -P 3
16.2.2. replacement_timeout
replacement_timeout 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 replacement_timeout value is
120 seconds.
To adjust replacement_timeout, open /etc/iscsi/iscsid.conf and edit the following line:
node.session.timeo.replacement_timeout = [replacement_timeout]
The 1 queue_if_no_path option in /etc/multipath.conf sets iSCSI timers to immediately
defer commands to the multipath layer (refer to
This setting prevents I/O errors from propagating to the application; because of this, you can set
replacement_timeout to 15-20 seconds.
By configuring a lower replacement_timeout, 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/multipath.conf instead of /etc/iscsi/iscsid.conf.
Important
Whether your considerations are failover speed or security, the recommended value for
replacement_timeout will depend on other factors. These factors include the network,
target, and system workload. As such, it is recommended that you thoroughly test any
new configurations to replacements_timeout before applying it to a mission-critical
system.

16.3. iSCSI Root

When accessing the root partition directly through a iSCSI disk, the iSCSI timers should be set so that
iSCSI layer has several chances to try to reestablish a path/session. In addition, commands should
not be quickly re-queued to the SCSI layer. This is the opposite of what should be done when dm-
multipath is implemented.
To start with, NOP-Outs should be disabled. You can do this by setting both NOP-Out interval and
timeout to zero. To set this, open /etc/iscsi/iscsid.conf and edit as follows:
node.conn[0].timeo.noop_out_interval = 0
node.conn[0].timeo.noop_out_timeout = 0
In line with this, replacement_timeout should be set to a high number. This will instruct the system
to wait a long time for a path/session to reestablish itself. To adjust replacement_timeout, open /
etc/iscsi/iscsid.conf and edit the following line:
28
Section 16.2, "iSCSI Settings With
dm-multipath").