Multipath and AutoYaST experiments

multipath.md

Facts

• When multipath is activated in a Linux system, ALL disks in such system are grouped into multipath devices. For example, if sda is an individual disk while sdb and sdc are actually part of the same multipath, the system will contain two multipath block devices (one for sdb+sdc and another one for sda only). That's represented in the targetmap by the corresponding CT_DISK and CT_DMMULTIPATH elements. When using libstorage-ng, all those disks and multipath devices are also represented in the devicegraph.
• With AutoYaST, multipath is only activated if start_multipath is true. The presence of a CT_DMMULTIPATH drive has no influence.
• Important: AutoYaST in SLE12-SPX will often select the disk with bios_id=0x80 for the first drive, no matter whether such drive is CT_DISK or CT_DMMULTIPATH and no matter whether the disk is part of a multipath device. See this code.
• Except the exception mentioned above, drives only match elements in the targetmap with its very own type (CT_MULTIPATH or CT_DISK).

So...

• With start_multipath

  • Only CT_DMMULTIPATH drives should be used, since you never want to use disks directly.
  • CT_DISK drives are useless, they will only match disks that are part of a multipath device.
  • Moreover, drives that were assigned to such disks will be ignored later.
  • Pitfall: a disk with bios_id=0x80 can match a CT_DMMULTIPATH drive, which mess everything up.

• Without start_multipath

  • Only CT_DISK drives should be used, there are no multipath devices in the system.
  • CT_DMMULTIPATH drives are useless, they SHOULD match with nothing.
  • Exception to CT_DMMULTIPATH matching nothing: if the first drive is CT_DMMULTIPATH and there is a disk with bios_id=0x80, that will match.

The Experiments we did (all of them without bios_id=0x80) if you are curious

You can use edd=off to prevent bios_id to introduce noise in your experiments, although it didn't work in some of my tests. You can also use EFI.

Martin's scenario - Only one multipath device and only one drive in profile

start_mp	type	MP activated	Device matched	Note
(missing)	CT_DISK	no	first individual disk - sda
(missing)	CT_DMMULTIPATH	no	none
true	CT_DISK	yes	first individual disk - sda	BootStorage err. Missing '/'
true	CT_DMMULTIPATH	yes	multipath device

Imo's scenario A - System with no multipath devices (2 disks)

start_mp	type (/ + /home)	MP activated	Devices matched
irrelevant	CT_DISK x2	no	individual disks, by position
irrelevant	CT_DMMULTIPATH x2	no	error. Why?
irrelevant	CT_DMMULTIPATH	no	first individual disk?

Imo's scenario B - System with 1 regular disk and 1 multipath (in that order?)

Important: when multipath is activated, the targetmap contains this

• /dev/mapper/QEMU_HARDDISK_0001 - CT_DMMULTIPATH - devices: (/dev/sdb, /dev/sdc)
• /dev/mapper/QEMU_HARDDISK_QM00001 - CT_DMMULTIPATH - devices: (/dev/sda)
• /dev/sda - CT_DISK
• /dev/sdb - CT_DISK
• /dev/sdc - CT_DISK

start_mp	type (/ + /home)	MP activated	Devices matched	Note
true	CT_DM x2	yes	QEMU_HARDDISK_0001 + QEMU_HARDDISK_QM00001
true	CT_DISK + CT_DM	yes	sda + QEMU_HARDDISK_0001	BootStorage err. Missing '/'
true	CT_DM + CT_DISK	yes	QEMU_HARDDISK_0001 + sda	/home (that was assigned to sda) was ignored
false	CT_DM x2	no?	none?
false	CT_DISK x2	no?	it works... but how it matches?

See also https://bugzilla.suse.com/show_bug.cgi?id=1135735 (to be turned into a similar table)