Installation guide

iSCSI Root
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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.
21.16.3. 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:
node.session.timeo.replacement_timeout = replacement_timeout
After configuring /etc/iscsi/iscsid.conf, you must perform a re-discovery of the affected
storage. This will allow the system to load and use any new values in /etc/iscsi/iscsid.conf.
For more information on how to discover iSCSI devices, refer to Section 21.12, “ Scanning iSCSI
Interconnects”.
Configuring Timeouts for a Specific Session
You can also configure timeouts for a specific session and make them non-persistent (instead of
using /etc/iscsi/iscsid.conf). To do so, run the following command (replace the variables
accordingly):
iscsiadm -m node -T target_name -p target_IP:port -o update -n
node.session.timeo.replacement_timeout -v $timeout_value
Important
The configuration described here is recommended for iSCSI sessions involving root partition
access. For iSCSI sessions involving access to other types of storage (namely, in systems that
use dm-multipath), refer to Section 21.16.2, “iSCSI Settings With dm-multipath”.