81 lines
2.3 KiB
Plaintext
81 lines
2.3 KiB
Plaintext
Dependancies :
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==============
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o libdevmapper : comes with device-mapper-XXXX.tar.gz
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See www.sistina.com
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This lib has been dropped in the multipath tree
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o libsysfs : comes with sysutils
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See ftp.kernel.org/pub/linux/utils/kernel/hotplug/
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How it works :
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==============
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Fill the all_paths array. Each path store this info :
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struct path {
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char dev[FILE_NAME_SIZE];
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char sg_dev[FILE_NAME_SIZE];
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struct scsi_idlun scsi_id;
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struct sg_id sg_id;
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int state;
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char wwid[WWID_SIZE];
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};
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scsi_id, sg_dev and sg_id are only really useful for 2.4
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kernels, for which SG cmnds must go through sg devs.
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In 2.5+ we have the nice opportunity to send SG cmnds
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through SCSI bdevs.
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For 2.4 compat, we pivot on idlun tupple to map sg devs
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to SCSI bdevs.
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2.4 does not do device enumeration, so we must scan a
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defined number of sg devs and scsi bdevs. Good enough.
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In 2.5+, we rely on libsysfs (sysutils) to access to
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sysfs device enums.
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the wwid is retrieved by a switch fonction. Only White
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Listed HW can filled this field. For now every FC array
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HW listed in kernel's devinfo.c is White Listed, assuming
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the WWID is stored is the SCSI-3 standard 0x83 EVPD page.
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When all_paths is filled, we coalesce the paths and store
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the result in mp array. Each mp is a struct like this :
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struct multipath {
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char wwid[WWID_SIZE];
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int npaths;
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int pindex[MAX_MP_PATHS];
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};
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When mp is filled, the device maps are fed to the kernel
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through libdevmapper.
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Notes :
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=======
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o On 2.4, make sure you have enough /dev/sg* nodes
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(/dev/MAKEDEV if necesary). Same goes for /dev/sd*
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o path coalescing relies on a path unique id being found.
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This unique id, lacking a standard method, is vendor
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specific. A switch function (get_unique_id) is present
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and an example function is provided for storageworks
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arrays (get_evpd_wwid). Feel free to enrich
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with hardware you have at hand :)
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o The kernel does NOT manage properly ghosts paths
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with StorageWorks HW. Seems nobody cares after a load
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of posts to linux-scsi.
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o 2.4.21 version of DM does not like even segment size.
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if you enconter pbs with this, upgrade DM.
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Credits :
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=========
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o Heavy cut'n paste from sg_utils. Thanks goes to D.
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Gilbert.
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o Light cut'n paste from dmsetup. Thanks Joe Thornber.
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o Greg KH for the nice sysfs API.
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