Linux Storage - Hot removal of a LUN

Linux Storage Solution · Config document · referenced from RDAC problems and LUN removal

Problem 2 of my notes: a LUN had to be removed from the running system without a reboot (hot remove). The notes collect three ways to say "delete" to the SCSI layer through sysfs, the command that shows which disk is which, a quoted English note about taking a path offline, a small script that prints the vendor and model of every block device, and, as the case actually done, the removal of all eight physical paths to the standby array DS5300-SITEB (array ID 1).

The commands

Removing a LUN by its disk name, where sd* is the disk device to be removed from the system. Here sdc and sdd, the two virtual disks of the standby array:

$ echo "1" > /sys/block/sdc/device/delete
$ echo "1" > /sys/block/sdd/device/delete

Removing a LUN by its SCSI address, where h is the HBA number, c the channel on the HBA, t the SCSI target ID and l the LUN. The notes keep the pattern, not a run of it:

$ echo 1 > /sys/class/scsi_device/h:c:t:l/device/delete

The device name, the HBA number, the channel, the target and the LUN are found with one listing. The notes keep no output for it:

$ ls -l /dev/disk/by-*

The notes then quote a note in English, which I give as it stands: "Take the path offline using echo offline > /sys/block/sda/device/state. This will cause any subsequent IO sent to the device on this path to be failed immediately. Device-mapper-multipath will continue to use the remaining paths to the device." It speaks of Device-mapper multipath, not of RDAC, and sda in it is an example name, not the sda of this host, which is LUN 1 of the main array.

The small script "who uses what": for every block device that has a vendor attribute, print its name, vendor and model. It is kept as it was in the notes, with its unclosed typographic quote:

for DSK in `ls /sys/block/`
do
if [ -e "/sys/block/${DSK}/device/vendor" ]
then
VENDOR=`cat /sys/block/${DSK}/device/vendor`
MODEL=`cat /sys/block/${DSK}/device/model`
echo “${DSK} ${VENDOR} ${MODEL}
fi
done

The paths leading to the array with ID 1, DS5300-SITEB:

$ mppUtil -g 1 | grep PathId

PathId: 77000002 (hostId: 0, channelId: 0, targetId: 2)
PathId: 77010002 (hostId: 1, channelId: 0, targetId: 2)
PathId: 77000003 (hostId: 0, channelId: 0, targetId: 3)
PathId: 77010003 (hostId: 1, channelId: 0, targetId: 3)

Deleting every path to the array with ID 1: two hosts, two targets, two LUNs, eight SCSI devices. The lines are kept exactly, with the 1> of the note above; the notes write them in four pairs, one pair per path, separated by blank lines, which the fence leaves out:

$ echo 1>/sys/bus/scsi/devices/0\:0\:2\:1/delete
$ echo 1>/sys/bus/scsi/devices/0\:0\:2\:2/delete
$ echo 1>/sys/bus/scsi/devices/1\:0\:2\:1/delete
$ echo 1>/sys/bus/scsi/devices/1\:0\:2\:2/delete
$ echo 1>/sys/bus/scsi/devices/0\:0\:3\:1/delete
$ echo 1>/sys/bus/scsi/devices/0\:0\:3\:2/delete
$ echo 1>/sys/bus/scsi/devices/1\:0\:3\:1/delete
$ echo 1>/sys/bus/scsi/devices/1\:0\:3\:2/delete

Reading it

The order of the eight lines follows the PathId list: first both LUNs of target 2 on host 0, then target 2 on host 1, then target 3 on hosts 0 and 1. What the notes do not hold: whether sdc and sdd were removed first (the first two lines) and the eight paths afterwards, or whether the two blocks are alternatives; a listing before and after; and whether the driver's mppBusRescan -d ("remove an unmapped or disconnected device", see mppBusRescan commands) was tried instead. The links section of the notes keeps a forum thread titled "RDAC capable of hot removing unmapped LUNs?", so the question was on the author's mind.

Checked against the Linux kernel 7.2 and Red Hat Enterprise Linux 10