[PATCH] Add multipath "extra" program from Christophe Varoqui, <christophe.varoqui@free.fr>

1 files changed, 82 insertions, 0 deletions

diff --git a/extras/multipath/README b/extras/multipath/README
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+++ b/extras/multipath/README
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+Dependancies :
+==============
+
+o libdevmapper : comes with device-mapper-XXXX.tar.gz
+  See www.sistina.com
+o libsysfs : comes with sysutils
+  See ftp.kernel.org/pub/linux/utils/kernel/hotplug/
+
+How it works :
+==============
+
+Fill the all_paths array. Each path store this info :
+
+struct path {
+        char dev[FILE_NAME_SIZE];
+        char sg_dev[FILE_NAME_SIZE];
+        struct scsi_idlun scsi_id;
+        struct sg_id sg_id;
+        int state;
+        char wwid[WWID_SIZE];
+};
+
+scsi_id, sg_dev and sg_id are only really useful for 2.4 
+kernels, for which SG cmnds must go through sg devs.
+In 2.5+ we have the nice opportunity to send SG cmnds
+through SCSI bdevs.
+
+For 2.4 compat, we pivot on idlun tupple to map sg devs
+to SCSI bdevs.
+
+2.4 does not do device enumeration, so we must scan a
+defined number of sg devs and scsi bdevs. Good enough.
+In 2.5+, we rely on libsysfs (sysutils) to access to
+sysfs device enums.
+
+the wwid is retrieved by a switch fonction. Only White
+Listed HW can filled this field. For now only 
+StorageWorks HW is White Listed. (See notes)
+
+When all_paths is filled, we coalesce the paths and store
+the result in mp array. Each mp is a struct like this :
+
+struct multipath {
+        char wwid[WWID_SIZE];
+        int npaths;
+        int pindex[MAX_MP_PATHS];
+};
+
+When mp is filled, the device maps are fed to the kernel
+through libdevmapper. Stale paths (failed TUR) are 
+discarded.
+
+Notes :
+=======
+
+o make sure you have enough /dev/sg* nodes
+  (/dev/MAKEDEV if necesary)
+
+o path coalescing relies on a path unique id being found.
+  This unique id, lacking a standard method, is vendor
+  specific. A switch function (get_unique_id) is present
+  and an example function is provided for storageworks
+  arrays (get_storageworks_wwid). Feel free to enrich
+  with hardware you have at hand :)
+
+o Something goes wrong with sd.o, qla2200 & dm-mod
+  refcounting : I can't unload these modules after exec.
+
+o The kernel does NOT manage properly ghosts paths
+  with StorageWorks HW. Seems nobody cares after a load
+  of posts to linux-scsi.
+
+o 2.4.21 version of DM does not like even segment size.
+  if you enconter pbs with this, upgrade DM.
+
+Credits :
+=========
+
+o Heavy cut'n paste from sg_utils. Thanks goes to D. 
+  Gilbert.
+o Light cut'n paste from dmsetup. Thanks Joe Thornber.
+o Greg KH for the nice sysfs API.