To provide a single device that can remain accessible if an I/O path fails or becomes saturated,
mdadm
includes an additional parameter to its
––level
option. This parameter —
multipath
—
directs the md layer in the Linux kernel to re-route I/O requests from one pathway to another in
the event of an I/O path failure.
To create a multipath device, edit the
/etc/mdadm.conf
file to define values for the
DEVICE
and
ARRAY
lines that reflect your hardware configuration.
Note
Unlike the previous RAID example (where each device specified in
/etc/mdadm.conf
must represent different physical disk drives), each device in
this file refers to the same shared disk drive.
The command used for the creation of a multipath device is similar to that used to create a
RAID device; the difference is the replacement of a RAID level parameter with the
multipath
parameter:
mdadm -C /dev/md0 --level=multipath --raid-devices=4 /dev/sda1 /dev/sdb1
/dev/sdc1 /dev/sdd1
Continue creating array? yes
mdadm: array /dev/md0 started.
Due to the length of the mdadm command line, it has been broken into two lines.
In this example, the hardware consists of one SCSI LUN presented as four separate SCSI
devices, each accessing the same storage by a different pathway. Once the multipath device
/dev/md0
is created, all I/O operations referencing
/dev/md0
are directed to
/dev/sda1
,
/dev/sdb1
,
/dev/sdc1
, or
/dev/sdd1
(depending on which path is currently active and
operational).
The configuration of
/dev/md0
can be examined more closely using the command
mdadm
--detail /dev/md0
to verify that it is, in fact, a multipath device:
/dev/md0:
Version : 00.90.00
Creation Time : Tue Mar
2 10:56:37 2004
Raid Level : multipath
Array Size : 3905408 (3.72 GiB 3.100 GB)
Raid Devices : 1
Total Devices : 4
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Tue Mar
2 10:56:37 2004
State : dirty, no-errors
Active Devices : 1
Multipath Storage
71
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