LWN: Comments on "Multipath support in the device mapper"

Multipath support in the device mapper

maddy — Mon, 07 Nov 2005 13:04:00 +0000

what is device mapper ? how it works ? why we need it ?
if any one knows kindly help me

LUNs and LUs

lutchann — Sat, 26 Feb 2005 18:22:23 +0000

I tend to think of "solution" as just a pretentious term for "thingy". Doing that word substitution in my head makes IT marketing literature somewhat more tolerable.

`actio' --> `action'

Max.Hyre — Fri, 25 Feb 2005 20:19:44 +0000

It seemed to be so simple a comment that I didn't really proofread it.

``Solution''s eating the heart out of technical discussion

Max.Hyre — Fri, 25 Feb 2005 20:15:50 +0000

Sadly, I saw this in actio earlier today. A colleague, a mostly-techno type, was describing a test setup to me. He really said, ``The client bridge can plug into both wireless and wired solutions''. When I asked whether he was trying to describe wireless and wired networks (it's even a syllable shorter), he allowed as how he was. :-(

LUNs and LUs

giraffedata — Fri, 25 Feb 2005 17:44:51 +0000

In most cases, it's just intellectually irritating when people call SCSI logical units LUNs (LUN = logical unit number). But when you're talking about a complex network like this with multiple paths, it's downright confusing. If there are 8 paths to a LU, the LU could have 8 different LUNs.

People started counting storage equipment by counting LUNs because it solved the ambiguity of what you consider one unit. Like counting spindles of disk or heads of sheep (even though you aren't actually interested in the spindles or heads themselves). Now, it doesn't solve any ambiguity and using a LUN as a metaphor for the function of a logical unit is just wrong.

Speaking of identifying units, I notice that you plug your fibre channel cables into a "solution." I wonder if that's something technical people would know better by an older name, such as "subsystem" or "network."

Multipath support in the device mapper

James — Thu, 24 Feb 2005 17:15:47 +0000

This has also been tested against IBM San Virtual Controller (SVC), where 8 data paths are
available to each LUN. Each (Linux) host has two physical fibre HBAs in them, each HBA
connecting to a separate fibre switch. Each switch in turn is connected to two (or more) nodes of
the IBM SVC solution. The SVC product virtualises real storage; it partitions the fibre network into
two parts (kind of like two vlans on an IP switch). In one side, we have a SAN controller, or several
SAN controllers (eg, IBM DS4100, or other manufacturers). On the other, we have the hosts. All
hosts talk to the SVC for access to the storage. SVC controls what goes where. It can stripe
across multiple SANs, and do on-line migration of data between SANs, replication, etc. Plus
online growth of LUNs. It also has gigs of memory to cache the I/O operations, so it is really fast
(all battery backed by its own required UPS). The SVC nodes themseleves are just 1U rackmount
boxes with loads of HBAs and these large UPS' attached.

We're quite happy with IBM SATA disk controllers (DS4100), expanded with EXP100 units. Each
chassis is 3.5T raw, and lots cheaper than SCSI. Using the SAN controller, create RAID1 or RAID5
arrays, which makes real LUNs (managed disks or mdisks in SVC lingo). SVN then takes those
LUNs, and stripes them up. You can then create virtual LUNs (vdisks) that the hosts see across
the 8 I/O paths that multipath here uses. So you then have large, expandable, on-line movable,
snapshottable (at multiple levels - LVM and within the SVC), HA disk.

Oh, and each 1U SVC host is running some form of Linux, supposedly.

Huge thanks to Alisdair et al. for their time on this code. Its way cool.