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Chapter 7. Managing multipathed volumes
You can manage multipath volumes by using the multipath
, dmsetup
, and multipathd
commands, which are provided by DM Multipath.
7.1. Resizing an online multipath device
You can resize an online multipath device and a file system using the multipad
and resize2fs
commands .
Procedure
- Resize your physical device.
Find the paths to the logical unit number (LUN):
# multipath -l
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:# echo 1 > /sys/block/path_device/device/rescan
Ensure that you run this command for each of the path devices. For example, if your path devices are
sda
,sdb
,sde
, andsdf
, run the following commands:# 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
Resize your multipath device:
# multipathd resize map multipath_device
Resize the file system, assuming no LVM or DOS partitions are used:
# resize2fs /dev/mapper/mpatha
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 added another path to the root file system, move your root file system 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 setchkconfig
for themultipathd
toon
:# mpathconf --enable
-
If the
find_multipaths
configuration parameter is not set toyes
, edit theblacklist
andblacklist_exceptions
sections of the/etc/multipath.conf
file, as described in Preventing devices from multipathing. Build a multipath device on top of the root device as soon as it is discovered. This command also ensures that
find_multipaths
allows the device, even if it only has one path.# multipath -a /dev/sdb` wwid '3600d02300069c9ce09d41c4ac9c53200' added
Replace /dev/sdb with the root device name.
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.date wwid: ignoring map
For example, if the WWID of the device is 3600d02300069c9ce09d41c4ac9c53200, you would see a line in the output such as the following:
# multipath Oct 21 09:37:19 | 3600d02300069c9ce09d41c4ac9c53200: ignoring map
Rebuild the
initramfs
file system withmultipath
:# dracut --force -H --add 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:
# multipath -l | grep 3600d02300069c9ce09d41c4ac9c53200 mpatha (3600d02300069c9ce09d41c4ac9c53200) dm-0 3PARdata,VV
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:# multipath -a /dev/sdb wwid '3600d02300069c9ce09d41c4ac9c53200' added
Replace /dev/sdb with the swap device name.
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:date wwid: ignoring map
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:
#
multipath
Oct 21 09:37:19 | 3600d02300069c9ce09d41c4ac9c53200: ignoring mapSet up an alias for the swap device in the
/etc/multipath.conf
file:multipaths { multipath { wwid WWID_of_swap_device alias swapdev } }
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:/dev/sdb2 swap swap defaults 0 0
Change the entry to the following:
/dev/mapper/swapdev swap swap defaults 0 0
Rebuild the initramfs file system with multipath:
# dracut --force -H --add 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:
# swapon -s Filename Type Size Used Priority /dev/dm-3 partition 4169724 0 -2
The file name should match the multipath swap device.
# readlink -f /dev/mapper/swapdev /dev/dm-3
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
.#
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)
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:# multipathd show maps name sysfs uuid mpathc dm-0 360a98000324669436c2b45666c567942
Some
multipathd
commands include aformat
option followed by a wildcard. Display a list of available wildcards with the following command:# multipathd show wildcards multipath format wildcards: %n name %w uuid %d sysfs ...
Display the multipath devices that
multipathd
is monitoring. Use wildcards to specify the shown fields:# multipathd show maps format "%n %w %d %s" name uuid sysfs vend/prod/rev mpathc 360a98000324669436c2b45666c567942 dm-0 NETAPP,LUN
Display the paths that
multipathd
is monitoring. Use wildcards to specify the shown fields:# multipathd show paths format "%n %w %d %s" target WWNN uuid dev vend/prod/rev 0x50001fe1500d2250 3600508b4001080520001e00011700000 sdb HP,HSV210
Display data in a raw format:
# multipathd show maps raw format "%n %w %d %s" mpathc 360a98000324669436c2b45666c567942 dm-0 NETAPP,LUN
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 on your system