Multipath

https://en.wikipedia.org/wiki/Linux_DM_Multipath

• https://linux.die.net/man/5/multipath.conf
• https://linux.die.net/man/8/multipath
• https://linux.die.net/man/8/multipathd

• mpathconf
  • mpathconf --enable --with_multipathd y
• multipath -l
• multipath -ll
  • multipath -ll DEVICE_NAME

• multipath -v2 or multipath -v3
• multipath -t

-t Display the currently used multipathd configuration

• multipathd show config

• Ubuntu: multipath-tools package
  • Binaries: multipath multipathd mpathpersist
• RHEL: device-mapper-multipath package

Config files:

• /etc/multipath.confhttps://manpages.debian.org/unstable/multipath-tools/multipath.conf.5.en.html
• /etc/multipath/wwids

Configuration file: /etc/multipath.conf

1. System level defaults (defaults): User can override system level defaults.
2. Blacklisted devices (blacklist): User specifies the list of devices that is not to be under the control of DM-MPIO.
3. Blacklist exceptions (blacklist_exceptions): Specific devices to be treated as multipath devices even if listed in the blacklist.
4. Storage controller specific settings (devices): User specified configuration settings will be applied to devices with specified "Vendor" and "Product" information.
5. Device specific settings (multipaths): Fine tune the configuration settings for individual LUNs.

Routing policies implemented: failover, multibus and group_by_serial

• failover : 1 path per priority group. IO thus get routed to one path only.
• multibus : 1 priority group containing all paths to the LU. Brings the maximum spreading, but assumes that all paths are excitable without penalty.
• group_by_serial : 1 priority group per storage controller (serial), paths through one controller are assigned to the associated PG. This policy applies to controllers that impose a latency penalty on LU management hand-over between a pair of redundant controllers.

To modify a policy for device major 8 and minor:

• multipath -p multibus -D 8 0

Readings

• Varoqui, C. (2010.) The Linux multipath implementation. Retrieved from http://christophe.varoqui.free.fr/refbook.html
• Changelog: https://git.opensvc.com/?p=multipath-tools/.git;a=log

Related commands

• blkid multipath -f multipath -w pvcreate

Activities

• Review no_path_retry configuration options in multipath.conf to avoid pvs to hang : https://www.suse.com/support/kb/doc/?id=7022310
• Multipath: remove multipath device unpresented to host: multipath -f
• Review source code: https://git.opensvc.com/gitweb.cgi?p=multipath-tools%2F.git
• Map some LUN to your system with multipath uninstalled and verify no dm devices are created. Install multipath and check again.

See also

• EMC powerpath
• kpartx
• Fibre Channel: HBA, multipath-tools, multipath, multipath -ll, /etc/multipath.conf multipathd rescan-scsi-bus.sh powermt Device mapper dmsetup ls,
• Block devices: blkid (UUID), lshw -class disk, hwinfo --disk, udevadm, rescan-scsi-bus.sh, LUN, hdparm, findfs, Add a new disk
• lsblk, pvscan lspci, lsscsi, smartctl, fdisk, parted, Kpartx, pvdiplay, pvs, udisksctl, /dev/sd, nvme list
• LVM: LVM errors LVM scan commands lkvmetad, Add a new disk

scan: pvscan vgscan

LVM: lvs lvmscan lvm fullreport lvmreport lvmconfig lvmdump lvmcheck lvm dumpconfig

Scan: pvs vgs lvs pvscan vgscan

PV: pvcreate pvremove pvscan pvremove pvscan pvs pvchange pvck pvdisplay pvresize

LV: lvdisplay lvcreate lvremove, lvresize, lvextend, lvreduce, lvrename

VG: vgdisplay,vgcreate,vgremove, vgextend, vgreduce, vgscan, vgchange, vgrename, vgcfgbackup, vgcfgrestore, vgimportclone, vgck

Check commands: pvck vgck

https://www.thegeekdiary.com/understanding-the-dm-multipath-configuration-file-etc-multipath-conf/

https://en.wikipedia.org/wiki/Linux_DM_Multipath