First cut at GEOM based multipath. This is an active/passive{/passive...}

arrangement that has no intrinsic internal knowledge of whether devices
it is given are truly multipath devices. As such, this is a simplistic
approach, but still a useful one.

The basic approach is to (at present- this will change soon) use camcontrol
to find likely identical devices and and label the trailing sector of the
first one. This label contains both a full UUID and a name. The name is
what is presented in /dev/multipath, but the UUID is used as a true
distinguishor at g_taste time, thus making sure we don't have chaos
on a shared SAN where everyone names their data multipath as "Fred".

The first of N identical devices (and N *may* be 1!) becomes the active
path until a BIO request is failed with EIO or ENXIO. When this occurs,
the active disk is ripped away and the next in a list is picked to
(retry and) continue with.

During g_taste events new disks that meet the match criteria for existing
multipath geoms get added to the tail end of the list.

Thus, this active/passive setup actually does work for devices which
go away and come back, as do (now) mpt(4) and isp(4) SAN based disks.

There is still a lot to do to improve this- like about 5 of the 12
recommendations I've received about it,  but it's been functional enough
for a while that it deserves a broader test base.

Reviewed by: pjd
Sponsored by: IronPort Systems
MFC: 2 months

1 files changed, 1 insertions, 0 deletions

diff --git a/sys/conf/NOTES b/sys/conf/NOTES
index 3174ad472515..e3096558fd0e 100644
--- a/sys/conf/NOTES
+++ b/sys/conf/NOTES
@@ -147,6 +147,7 @@ options 	GEOM_JOURNAL		# Journaling.
 options 	GEOM_LABEL		# Providers labelization.
 options 	GEOM_MBR		# DOS/MBR partitioning
 options 	GEOM_MIRROR		# Disk mirroring.
+options 	GEOM_MULTIPATH		# Disk multipath
 options 	GEOM_NOP		# Test class.
 options 	GEOM_PART_APM		# Apple partitioning
 options 	GEOM_PART_GPT		# GPT partitioning