Multipath support in the device mapper
Posted Feb 24, 2005 17:15 UTC (Thu) by James
Parent article: Multipath support in the device mapper
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.
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