Configuring Multipathing
The procedure in this
section demonstrates how to set up a simple multipath configuration.
To configure
multipathing on a server with access to SAN-attached storage:
1.
Install the device-mapper-multipath package:
#
yum
install device-mapper-multipath
2.
You can now choose one
of two configuration paths:
§ To set up a basic standby failover configuration
without editing the /etc/multipath.conf configuration file, enter the following command:
#
mpathconf
--enable --with_multipathd y
This command also starts the multipathd service and configures the service to start
after system reboots.
Skip the remaining steps of this procedure.
§ To edit /etc/multipath.conf and set up a more complex configuration such as
active/active, follow the remaining steps in this procedure.
Initialize the /etc/multipath.conf file:
#
mpathconf
--enable
Edit /etc/multipath.conf and define defaults, blacklist, blacklist_exceptions, multipaths, and devices sections as required, for example:
defaults {
udev_dir /dev
polling_interval 10
path_selector "round-robin 0"
path_grouping_policy multibus
getuid_callout "/lib/udev/scsi_id --whitelisted
--device=/dev/%n"
prio alua
path_checker readsector0
rr_min_io 100
max_fds 8192
rr_weight priorities
failback immediate
no_path_retry fail
user_friendly_names yes
}
blacklist {
# Blacklist by WWID
wwid "*"
# Blacklist by device name
devnode
"^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
# Blacklist by device type
device {
vendor
"COMPAQ "
product
"HSV110 (C)COMPAQ"
}
}
blacklist_exceptions {
wwid
"3600508b4000156d700012000000b0000"
wwid
"360000970000292602744533032443941"
}
multipaths {
multipath {
wwid
3600508b4000156d700012000000b0000
alias blue
path_grouping_policy multibus
path_checker readsector0
path_selector "round-robin 0"
failback manual
rr_weight priorities
no_path_retry 5
}
multipath {
wwid
360000970000292602744533032443941
alias green
}
}
devices {
device {
vendor "SUN"
product "(StorEdge 3510|T4"
path_grouping_policy multibus
getuid_callout "/sbin/scsi_id --whitelisted
--device=/dev/%n"
path_selector "round-robin 0"
features "0"
hardware_handler "0"
path_checker directio
prio const
rr_weight uniform
rr_min_io 1000
}
}
The sections have the following purposes:
defaults
Defines default multipath settings, which can be
overridden by settings in the devices section, and which in turn can be overridden by
settings in themultipaths section.
blacklist
Defines devices that are excluded from multipath
topology discovery. Blacklisted devices cannot subsumed by a multipath device.
The example shows the three ways that you can
use to exclude devices: by WWID (wwid), by device name (devnode), and by device type (device).
blacklist_exceptions
Defines devices that are included in multipath
topology discovery, even if the devices are implicitly or explicitly listed in
the blacklist section.
multipaths
Defines settings for a multipath device that is
identified by its WWID.
The alias attribute specifies the name of the multipath device as it will
appear in /dev/mapper instead of a name based on either the WWID or
the multipath group number.
To obtain the WWID of a SCSI device, use the scsi_id command:
#
scsi_id
--whitelisted --replace-whitespace --device=device_name
devices
Defines settings for individual types of storage
controller. Each controller type is identified by the vendor, product, and optional revision settings, which must match the information in sysfs for the device.
You can find details of the storage arrays that
DM-Multipath supports and their default configuration values in /usr/share/doc/device-mapper-multipath-version/multipath.conf.defaults, which you can use as the basis for entries in /etc/multipath.conf.
To add a storage device that DM-Multipath does
not list as being supported, obtain the vendor, product, and revision
information from the vendor,model, and rev files under /sys/block/device_name/device.
The following entries in /etc/multipath.conf would be appropriate for setting up
active/passive multipathing to an iSCSI LUN with the specified WWID.
defaults
{
user_friendly_names yes
getuid_callout "/bin/scsi_id --whitelisted
--replace-whitespace --device=/dev/%n”
}
multipaths
{
multipath {
wwid 360000970000292602744533030303730
}
}
In this standby failover configuration, I/O
continues through a remaining active network interface if a network interfaces
fails on the iSCSI initiator.
For more information about configuring entries
in /etc/multipath.conf, refer to the multipath.conf(5) manual page.
Start the multipathd service and configure the service to start after
system reboots:
# systemctl start
multipathd
#
systemctl
enable multipathd
Multipath devices are
identified in /dev/mapper by their World Wide Identifier (WWID), which is
globally unique. Alternatively, if you set the value ofuser_friendly_names to yes in the defaults section of /etc/multipath.conf or by specifying the --user_friendly_names
n option to mpathconf, the device is named mpathN where N is the multipath group number. An alias attribute in the multipaths section of /etc/multipath.conf specifies the name of the multipath device
instead of a name based on either the WWID or the multipath group number.
You can use the multipath
device in /dev/mapper to reference the storage in the same way as you
would any other physical storage device. For example, you can configure it as
an LVM physical volume, file system, swap partition, Automatic Storage
Management (ASM) disk, or raw device.
To display the status of
DM-Multipath, use the mpathconf command, for example:
# mpathconf
multipath is enabled
find_multipaths is
enabled
user_friendly_names is
enabled
dm_multipath modules is
loaded
multipathd is running
To display the current
multipath configuration, specify the -ll option to the multipath command, for example:
# multipath -ll
mpath1(360000970000292602744533030303730)
dm-0 SUN,(StorEdge 3510|T4
size=20G features=‘0’
hwhandler=‘0’ wp=rw
|-+- policy=‘round-robin
0’ prio=1 status=active
| ‘- 5:0:0:2 sdb
8:16 active ready running
‘-+- policy=‘round-robin
0’ prio=1 status=active
‘- 5:0:0:3 sdc 8:32 active ready running
In this example, /dev/mapper/mpath1 subsumes two paths (/dev/sdb and /dev/sdc) to 20 GB of storage in an active/active configuration using
round-robin I/O path selection. The WWID that identifies the storage is 360000970000292602744533030303730 and the name of the multipath device under sysfs is dm-0.
If you edit /etc/multipath.conf, restart the multipathd service to make it re-read the file:
# systemctl restart
multipathd
For more information,
see the mpathconf(8), multipath(8), multipathd(8), multipath.conf(5), and scsi_id(8) manual pages.
