Document iscsi and multipath implementations.

1 files changed, 143 insertions, 0 deletions

diff --git a/docs/multipath.txt b/docs/multipath.txt
new file mode 100644
index 000000000..e8af24ec9
--- /dev/null
+++ b/docs/multipath.txt
@@ -0,0 +1,143 @@
+======================
+Multipath and Anaconda
+======================
+
+
+Introduction
+------------
+
+If there are two block devices in your /dev for which udev reports the same
+'ID_SERIAL' then you can create a certain device mapper device which arbitrarily
+uses those devices to access the physical device. And that is Multipath [1].
+
+For instance, suppose there are
+
+/dev/sda, with ID_SERIAL of 20090ef12700001d2, and
+/dev/sdb, with the same ID_SERIAL.
+
+Those are probably some adapters in the system that just connect your box to a
+storage area network (SAN) somewhere. There are perhaps two cables, one for sda,
+one for sdb, and if one of the cables gets cut the other can still transmit
+data. Normally the system won't recognize that sda and sdb have this special
+relation to each other, but by creating a suitable device map using multipath
+tools [2] we can create a DM device /dev/mapper/mpatha and use it for storing
+and retrieving data.
+
+The device mapper then automatically routes IO requests to /dev/mapper/mpatha to
+either sda or sdb depending on the load of the line or network congestion on the
+particular network etc.
+
+The nomenclature I will use here is:
+
+- 'multipath device' for the smart /dev/mapper/mpathX device.
+- 'multipath member device' for the '/dev/sdX' devices. Also 'a path'.
+
+
+What is expected from Anaconda
+------------------------------
+
+Anaconda is expected to:
+- detect that there are multipath devices present
+- coalesce all relevant (e.g. exclusiveDisks) multipath devices.
+- only let the user interact with the multipath devices in filtering,
+  cleardiskssel and partition screen, that is once we know 'sdc' and 'sdd' are
+  part of 'mpathb' show only 'mpathb' and never the paths.
+- install bootloader and boot from an mpath device
+- make it happen so all the multipath devices (carrying or not the root
+  filesystem) we used for installation are correctly coalesced in the booted
+  system. This is achieved by generating a suitable /etc/multipath.conf and
+  writing it into sysroot.
+- be able to refer to mpath devices from kickstart, either by name like 'mpatha'
+  or by their id like 'disk/by-id/scsi-20090ef12700001d2'
+
+
+How Anaconda handles multipath
+------------------------------
+
+To detect presence of multipath devices we rely on multipath tools. The same we
+do for coalescing, see pyanaconda/storage/devicelibs/mpath.py, the file that
+provides some abstraction from mpath tools. During the device scan we use the
+'multipath -d' output to find out what devices are going to end up as multipath
+members. The MultipathTopology object also enhances the multipath member's udev
+dictionaries with 'ID_FS_TYPE' set to 'multipath_member' (yes, this is a hack
+surviving from the original mpath implementation, and righteous is he who
+eradicates it). This information is picked up by DeviceTree when populating
+itself. Meaning, if 'sda' and 'sdb' are multipath member devices DeviceTree
+gives them MultipathMember format and creates one MultipathDevice for them (we
+know its name from 'multipath -d'). We end up with:
+
+DiskDevice 'sda', format 'MultipathMember'
+DiskDevice 'sdb', format 'MultipathMember'
+MultipathDevice 'mpatha', parents are 'sda' and 'sdb'.
+
+From then on, Anaconda only deals with the MultipathDevice and generally leaves
+anything with 'MultipathMember' format alone (understand, this is an inert
+format that really is not there but we use it just to mark the device as
+"useless beyond a multipath member", kind of like MDRaidMember).
+
+Partition happens over the multipath device and during the preinstallconfig step
+/mnt/sysimage/etc/multipath.conf is created and filled with information about
+the coalesced devices. This is handled in the Storage.write() method. It is
+important this file and /etc/multipath/wwids (autogenerated by mpath tools)
+make it to the sysimage before the dracut image is generated.
+
+
+Debugging multipath bugs
+------------------------
+
+Unlike with iSCSI, to reproduce a multipath bug one does not need the same
+specific hardware as the reporter. Just found any box connected to a multipathed
+SAN and you are fine (at the moment, connecting to the same iSCSI target through
+its IPv4 and IPv6 address also produces a multipathed device).
+
+On top of that, much of the necessary information is already included in the
+anaconda logs or can be easily extracted from the reporter. The things to
+particularly look at are:
+
+- storage.log, the output around 'devices to scan for multipath' and 'devices
+  post multipath scan'. The latter shows a triple with regular disks, disks
+  comprising multipath devices and partitions. This helps you quickly find out
+  what the target system is about.
+
+- this information is also in program.log's calls to 'multipath' [3]. If mpath
+  devices are mysteriously appearing/disappearing between filtering and
+  partitioning screens look at those. 'multipath -ll' is called to display
+  currently coalesced mpath devices, 'multipath -d' is called to show the mpath
+  devices that would be coalesced if we ran 'multipath' now. This is exploited
+  by the device filtering screen.
+
+
+Future of multipath in Anaconda
+-------------------------------
+
+Overall as of RHEL6.2, the shape of multipath in Anaconda is good and what's
+more important it is flexible enough to sustain new RFEs and bugs. Those are
+however bugs that I expect to appear sometime soon:
+
+- enable or disable mpath_friendly_names in kickstart. Disabling friendly names
+  just means the mpath devices are called by their wwid,
+  e.g. /dev/mapper/360334332345343234, not '/dev/mapper/mpathc'. This is
+  straightforward to implement.
+- extend support for mpath devices in kickstart in general. Currently mpath
+  devices should be accepted in most commands but I am sure there will be corner
+  cases. Difficulty medium.
+- [rawhide] stop extending the udev info dictionary with 'ID_FS_TYPE' and
+  'ID_MPATH_NAME'. Doing it this way is asking for the trouble if a dictionary
+  of particular mpath device is reloaded from udev without running it through
+  the MultipathTopology object as it will miss those entries (and DeviceTree
+  depends on them a lot). Difficulty hard, but includes a lot of pleasant
+  refactoring.
+- Improve support for multipathing iSCSI devices. Someone might ask for it one
+  day (in fact, with the NIC bounding they already did), and it will make mpath
+  debugging possible on any virt machine with multiple virt NICs.
+
+
+
+[1] http://akozumpl.fedorapeople.org/archive/Multipass.jpg
+[2] http://christophe.varoqui.free.fr/
+[3] 'man 8 multipath'
+
+
+
+---
+Red Hat Author(s): Ales Kozumplik <akozumpl@redhat.com>