Chapter 7. Managing multipathed volumes The following are a few commands provided by DM Multipath, which you can use to manage multipath volumes: multipath dmsetup multipathd 7.1. Resizing an online multipath device If you need to resize an online multipath device, use the following procedure. Procedure Resize your physical device. Execute the following command to find the paths to the logical unit number (LUN): Resize your paths. For SCSI devices, writing a 1 to the rescan file for the device causes the SCSI driver to rescan, as in the following command: Ensure that you run this command for each of the path devices. For example, if your path devices are sda , sdb , sde , and sdf , you would run the following commands: Resize your multipath device: Resize the file system (assuming no LVM or DOS partitions are used): 7.2. Moving a root file system from a single path device to a multipath device If you have installed your system on a single-path device and later add another path to the root file system, you will need to move your root file system to a multipathed device. See the following procedure for moving from a single-path to a multipathed device. Prerequisites You have installed the device-mapper-multipath package. Procedure Create the /etc/multipath.conf configuration file, load the multipath module, and enable the multipathd systemd service: Execute the following command to create the /etc/multipath.conf configuration file, load the multipath module, and set chkconfig for the multipathd to on : If the find_multipaths configuration parameter is not set to yes , edit the blacklist and blacklist_exceptions sections of the /etc/multipath.conf file, as described in Preventing devices from multipathing . In order for multipath to build a multipath device on top of the root device as soon as it is discovered, enter the following command. This command also ensures that find_multipaths allows the device, even if it only has one path. For example, if the root device is /dev/sdb , enter the following command. Confirm that your configuration file is set up correctly by executing the multipath command and search the output for a line of the following format. This indicates that the command failed to create the multipath device. For example, if the WWID of the device is 3600d02300069c9ce09d41c4ac9c53200 , you would see a line in the output such as the following: Rebuild the initramfs file system with multipath : Shut the machine down. Boot the machine. Make the other paths visible to the machine. Verification Check whether the multipath device is created by running the following command: 7.3. Moving a swap file system from a single path device to a multipath device By default, swap devices are set up as logical volumes. This does not require any special procedure for configuring them as multipath devices as long as you set up multipathing on the physical volumes that constitute the logical volume group. If your swap device is not an LVM volume, however, and it is mounted by device name, you might need to edit the /etc/fstab file to switch to the appropriate multipath device name. Procedure Add the WWID of the device to the /etc/multipath/wwids file: For example, if the root device is /dev/sdb , enter the following command. Confirm that your configuration file is set up correctly by executing the multipath command and search the output for a line of the following format: This indicates that the command failed to create the multipath device. For example, if the WWID of the device is 3600d02300069c9ce09d41c4ac9c53200, you would see a line in the output such as the following: Set up an alias for the swap device in the /etc/multipath.conf file: Edit the /etc/fstab file and replace the old device path to the root device with the multipath device. For example, if you had the following entry in the /etc/fstab file: Change the entry to the following: Rebuild the initramfs file system with multipath: Shut the machine down. Boot the machine. Make the other paths visible to the machine. Verification Verify if the swap device is on the multipath device: For example: The file name should match the multipath swap device. 7.4. Determining device mapper entries with the dmsetup command You can use the dmsetup command to find out which device mapper entries match the multipathed devices. Procedure Display all the device mapper devices and their major and minor numbers. The minor numbers determine the name of the dm device. For example, a minor number of 3 corresponds to the multipathed device /dev/dm-3 . 7.5. Administering the multipathd daemon The multipathd commands can be used to administer the multipathd daemon. Procedure View the default format for the output of the multipathd show maps command: Some multipathd commands include a format option followed by a wildcard. Display a list of available wildcards with the following command: Display the multipath devices that multipathd is monitoring. Use wildcards to specify the shown fields: Display the paths that multipathd is monitoring. Use wildcards to specify the shown fields: Display data in a raw format: In raw format, no headers are printed and the fields are not padded to align the columns with the headers. This output can be more easily used for scripting. Additional resources multipathd (8) man page

Commands:
multipath -l
echo 1 > /sys/block/path_device/device/rescan
echo 1 > /sys/block/sda/device/rescan echo 1 > /sys/block/sdb/device/rescan echo 1 > /sys/block/sde/device/rescan echo 1 > /sys/block/sdf/device/rescan
multipathd resize map multipath_device
resize2fs /dev/mapper/mpatha
yum install device-mapper-multipath
mpathconf --enable
multipath -a root_devname
multipath -a /dev/sdb wwid '3600d02300069c9ce09d41c4ac9c53200' added
date wwid : ignoring map
multipath Oct 21 09:37:19 | 3600d02300069c9ce09d41c4ac9c53200: ignoring map
dracut --force -H --add multipath
multipath -l | grep 3600d02300069c9ce09d41c4ac9c53200 mpatha (3600d02300069c9ce09d41c4ac9c53200) dm-0 3PARdata,VV
multipath -a swap_devname
multipath -a /dev/sdb wwid '3600d02300069c9ce09d41c4ac9c53200' added
date wwid : ignoring map
multipath Oct 21 09:37:19 | 3600d02300069c9ce09d41c4ac9c53200: ignoring map
multipaths { multipath { wwid WWID_of_swap_device alias swapdev } }
/dev/sdb2 swap swap defaults 0 0
/dev/mapper/swapdev swap swap defaults 0 0
dracut --force -H --add multipath
swapon -s
swapon -s Filename Type Size Used Priority /dev/dm-3 partition 4169724 0 -2
readlink -f /dev/mapper/swapdev /dev/dm-3
dmsetup ls mpathd (253:4) mpathep1 (253:12) mpathfp1 (253:11) mpathb (253:3) mpathgp1 (253:14) mpathhp1 (253:13) mpatha (253:2) mpathh (253:9) mpathg (253:8) VolGroup00-LogVol01 (253:1) mpathf (253:7) VolGroup00-LogVol00 (253:0) mpathe (253:6) mpathbp1 (253:10) mpathd (253:5)
multipathd show maps name sysfs uuid mpathc dm-0 360a98000324669436c2b45666c567942
multipathd show wildcards multipath format wildcards: %n name %w uuid %d sysfs
multipathd show maps format "%n %w %d %s" name uuid sysfs vend/prod/rev mpathc 360a98000324669436c2b45666c567942 dm-0 NETAPP,LUN
multipathd show paths format "%n %w %d %s" target WWNN uuid dev vend/prod/rev 0x50001fe1500d2250 3600508b4001080520001e00011700000 sdb HP,HSV210
multipathd show maps raw format "%n %w %d %s" mpathc 360a98000324669436c2b45666c567942 dm-0 NETAPP,LUN