Installation guide

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
22.3.2. Creat ing a Mult ipat h Device Wit h 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
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:
Red Hat Ent erprise Linux 5 Inst allat ion G uide
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