More at http://christophe.varoqui.free.fr/wiki/wakka.php?wiki=FAQ

1. How to set up System-on-multipath ?
======================================

prerequisite : udev and multipath-tools installed

here are the steps on a Debian SID system :

* add dm-mpath and dm-multipath to /etc/mkinitrd/modules
* copy $tools_dir/multipath/0[12]_* to /etc/mkinitrd/scripts
* define a friendly alias for your multipathed root disk
  (in /etc/multipath.conf). Example : "system"
* enable busybox in /etc/mkinitrd/mkinitrd.conf and set ROOT
  to any valid block-device (but not a /dev/dm-* one, due to
  an mkintrd misbelief that all dm-* are managed by LVM2)
* run mkinitrd
* in /boot/grub/menu.lst, define the root= kernel parameter
  Example : root=/dev/system1
* modify /etc/fstab to reference /dev/system* partitions

At reboot, you should see some info like :

path /dev/sda : multipath system
...
gpt: 0 slices
dos: 5 slices
reduced size of partition #2 to 63
Added system1 : 0 70685937 linear /dev/system 63
Added system2 : 0 63 linear /dev/system 7068600
Added system5 : 0 995967 linear /dev/system 70686063
...

2. How does it compare to commercial product XXX ?
==================================================

Here are a few distinctive features :

* you can mix HBA models, even different vendors, different speed ...
* you can mix storage controllers on your SAN, and access them all, applying
  different path grouping policy
* completely event-driven model : no administration burden if you accept the
  default behaviours
* extensible : you can plug your own policies if the available ones don't fill
  your needs
* supports root FS on multipathed SAN
* free, open-source software

3. LVM2 doesn't see my multipathed devices as PV, what's up ?
=============================================================

By default, lvm2 does not consider device-mapper block devices (such as a
dm-crypt device) for use as physical volumes.

In order to use a dm-crypt device as an lvm2 pv, add this line to the
devices block in /etc/lvm/lvm.conf:

types = [ "device-mapper", 16 ]

If /etc/lvm/lvm.conf does not exist, you can create one based on your 
current/default config like so:

lvm dumpconfig > /etc/lvm/lvm.conf

(tip from Christophe Saout)

4. I see a lot of "io_setup failed" message using the directio checker
======================================================================

The directio path checker makes use of the asynchronous I/O API (aio) provided
by modern Linux systems. Asynchronous I/O allows an application to submit I/O
requests asynchronously and be notified later of their completion status. To
allow this, we must allocate an asynchronous I/O context (an object of type
aio_context_t) and this task is handled by the io_setup system call.

A system wide limit on the number of AIO contexts that may be active
simultaneously is imposed via the aio-max-nr sysctl parameter.

Once this limit has been reached further calls to io_setup will fail with the
error number EAGAIN leading to the "io_setup failed" messages seen for e.g. when
running "multipath -ll".

To avoid this problem the number of available aio contexts should be increased
by setting the aio-max-nr parameter. This can be set on a one-time basis via the
/proc file system, for e.g.:

        # echo 131072 > /proc/sys/fs/aio-max-nr

Doubles the number of available contexts from the default value of 65536.

To make this setting persistent a line may be added to /etc/sysctl.conf:

        fs.aio-max-nr = 131072

Consult appropriate application and operating system tuning recommendations for
guidance on appropriate values for this parameter.