[Ilugc] help with device mapper

Fri Jul 17 13:31:27 IST 2009

On Fri, Jul 17, 2009 at 12:00 PM, rajasekhar kuppa
<rajsekhar.kvs at gmail.com> wrote:
> The specifics i am looking are for multipathing across 2 disk arrays  and a
> m looking for at least some of the features of Powerpath/ DMP
>  from the FLOSS world

Multi-path is handled using 2 components: MPIO support from OS, and device (or
SAN controller) specific driver (aka DSM).

Linux requires the device mapper tools to handle multipathing inside the
kernel.  This is provided by the device mapper kernel modules.

The DSM is responsible for health checking of the IO path, you require the
appropriate DSMs for each SAN backend (eg EMC, HDS, etc).  DSM for several
storage vendors is already available in /sbin/mpath*.  The FOSS DSMs
provide round-robin, health checking, and group priority.  Vendor provided
DSMs (eg Powerpath) provide additional features, but don't enjoy community
backing.  I've used vendor provided DSMs (Powerpath, HDLM,etc) on Windows
only, and the FOSS/native drivers on Linux/Solaris.

Using the DSMs, a user space multipathd daemon periodically health checks the
LUNs and activates/removes the paths as necessary.

Here is the sample multipath output from one of my boxes:

[root at treble sbin]# ./multipath -l
prod03 (360060e801082e30004d4247000000003) dm-0 HITACHI,DF600F
[size=2.0T][features=0][hwhandler=0][rw]
\_ round-robin 0 [prio=0][active]
 \_ 2:0:0:3 sdh 8:112 [active][undef]
\_ round-robin 0 [prio=0][enabled]
 \_ 4:0:0:3 sdg 8:96  [active][undef]

This server is running FOSS HDS dsm and shows first path as
"active" and second path as "enabled".

> The specifics i am looking are for multipathing across 2 disk arrays  and a
> m looking for at least some of the features of Powerpath/ DMP
> from the FLOSS world

I'm not sure if it's advisable to multipath across two different SAN arrays.
At least I've never deployed anything in production like this!  Multipath
is typically done to a single SAN system with multiple controllers/fabrics/HBAs.