Multiple paths to storage devices can provide connection redundancy, failover capability, load balancing, and improved performance. Device-Mapper Multipath (DM-Multipath) is a multipathing tool that allows you to represent multiple I/O paths between a server and a storage device as a single path.
You would be most likely to configure multipathing with a system that can access storage on a Fibre Channel-based storage area network (SAN). You can also use multipathing on an iSCSI initiator if redundant network connections exist between the initiator and the target.
Figure 17.2 shows a simple DM-Multipath configuration where two I/O paths are configured between a server and a disk on a SAN-attached storage array:
Between host bus adapter
hba1on the server and controllerctrl1on the storage array.Between host bus adapter
hba2on the server and controllerctrl2on the storage array.
Without DM-Multipath, the system treats each path as being
separate even though it connects the server to the same storage
device. DM-Multipath creates a single multipath device,
/dev/mapper/mpath,
that subsumes the underlying devices, N/dev/sdc
and /dev/sdf.
You can configure the multipathing service
(multipathd) to handle I/O from and to a
multipathed device in one of the following ways:
- Active/Active
I/O is distributed across all available paths, either by round-robin assignment or dynamic load-balancing.
- Active/Passive (standby failover)
I/O uses only one path. If the active path fails, DM-Multipath switches I/O to a standby path. This is the default configuration.
DM-Multipath can provide failover in the case of path failure, such as in a SAN fabric. Disk media failure must be handled by using either a software or hardware RAID solution.