Installation guide

iSCSI Settings With dm-multipath
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2. This command will return Blocked when the remote port (along with devices accessed through
it) are blocked. If the remote port is operating normally, the command will return Online.
3. If the problem is not resolved within dev_loss_tmo seconds, the rport and devices will be
unblocked and all I/O running on that device (along with any new I/O sent to that device) will be
failed.
Procedure 21.6. Changing dev_loss_tmo
• To change the dev_loss_tmo value, echo in the desired value to the file. For example, to set
dev_loss_tmo to 30 seconds, run:
echo 30 > /sys/class/fc_remote_port/rport-H:B:R/dev_loss_tmo
For more information about dev_loss_tmo, refer to Section 21.1.1, “Fibre Channel API”.
When a device is blocked, the fibre channel class will leave the device as is; i.e. /dev/sdx will remain
/dev/sdx. This is because the dev_loss_tmo expired. If the link problem is fixed at a later time,
operations will continue using the same SCSI device and device node name.
Fibre Channel: remove_on_dev_loss
If you prefer that devices are removed at the SCSI layer when links are marked bad (i.e. expired
after dev_loss_tmo seconds), you can use the scsi_transport_fc module parameter
remove_on_dev_loss. When a device is removed at the SCSI layer while remove_on_dev_loss
is in effect, the device will be added back once all transport problems are corrected.
Warning
The use of remove_on_dev_loss is not recommended, as removing a device at the SCSI
layer does not automatically unmount any file systems from that device. When file systems from
a removed device are left mounted, the device may not be properly removed from multipath or
RAID devices.
Further problems may arise from this if the upper layers are not hotplug-aware. This is because
the upper layers may still be holding references to the state of the device before it was originally
removed. This can cause unexpected behavior when the device is added again.
21.16.2. 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.