Creating A Multipath Device With Mdadm - Red Hat ENTREPRISE LINUX 5 Installation Manual

Chapter 22. Additional Information for IBM System z Users
Version ­: 00.90.00
Creation Time ­: Mon Mar
Raid Level ­: raid0
Array Size ­: 15621632 (14.90 GiB 15.100 GB)
Raid Devices ­: 4
Total Devices ­: 4
Preferred Minor ­: 0
Persistence ­: Superblock is persistent
Update Time ­: Mon Mar
State ­: dirty, no­errors
Active Devices ­: 4
Working Devices ­: 4
Failed Devices ­: 0
Spare Devices ­: 0
Chunk Size ­: 64K
Number Major
0 8
1 8
2 8
3 8
UUID ­: 25c0f2a1:e882dfc0:c0fe135e:6940d932
Events ­: 0.1

22.3.2. 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 ­­­raid­devices=4 ­/dev/sda1 ­/dev/sdb1
­/dev/sdc1 ­/dev/sdd1
Continue creating array? yes
236
1 13:49:10 2004
1 13:49:10 2004
Minor RaidDevice State
1 0 active sync
17 1 active sync
33 2 active sync
49 3 active sync
­/dev/sda1
­/dev/sdb1
­/dev/sdc1
­/dev/sdd1