Red Hat Enterprise Linux Installation Guide 5.2 - linux.meuhobby.com

Multipath Storage
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0
Chunk Size : 64K
Number Major Minor RaidDevice State
0 8 1 0 active sync /dev/sda1
1 8 17 1 active sync /dev/sdb1
2 8 33 2 active sync /dev/sdc1
3 8 49 3 active sync /dev/sdd1
UUID : 25c0f2a1:e882dfc0:c0fe135e:6940d932
Events : 0.1
3.2. Creating a Multipath Device With mdadm
In addition to creating RAID arrays, mdadm can also be used to take advantage of hardware
supporting more than one I/O path to individual SCSI LUNs (disk drives). The goal of multipath
storage is continued data availability in the event of hardware failure or individual path
saturation. Because this configuration contains multiple paths (each acting as an independent
virtual controller) accessing a common SCSI LUN (disk drive), the Linux kernel detects each
shared drive once "through" each path. In other words, the SCSI LUN (disk drive) known as
/dev/sda may also be accessible as /dev/sdb, /dev/sdc, and so on, depending on the
specific configuration.
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
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