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56 Appendix F. Additional Information for S/390 and zSeries Users

Update Time : Mon Mar 1 13:49:10 2004

State : dirty, no-errors

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

F.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

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).

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