Red Hat Enterprise Linux 5 Administration Unleashed

184CHAPTER 7Managing StorageFailed Devices : 0Spare Devices : 1Checksum : ee90b526 - correctEvents : 0.5Number Major Minor RaidDevice Statethis 1 8 6 1 active sync /dev/sda60 0 0 0 0 removed1 1 8 6 1 active sync /dev/sda62 2 8 7 2 active sync /dev/sda73 3 8 8 3 spare /dev/sda8Using MD MultipathThe hard drives in a system are connected to the rest of the system hardware via a diskcontroller. If the controller fails, the system can no longer communicate with the drivesconnected to it. However, some systems offer multipath disk access in which more thanone controller is connected to the disks. If the active controller fails, a spare one replacesit, allowing continued access to the storage attached.An example usage of MD Multipath is when a system is connected to a storage areanetwork (SAN) via Fiber Channel Protocol or Cards. The multipath device can representone interface that connects to the SAN using multiple physical cables. If one or more ofthe physical connections stops working or gets disconnected, the other physical cables arestill active, and the storage is still accessible.The Linux kernel offers Multiple Device (MD) Multipathing via its software RAID feature.MD Multipathing allows a device to be set up with multiple spares so that if the activedevice fails, I/O requests do not fail. If the active partition fails, the kernel activates one ofthe spare partitions as the active one.To set up an MD Multipath device:mdadm --create /dev/mdX --level=multipath --raid-devices= For example, use the following to set up /dev/md0 with three drives, two of whichbecome spares:mdadm --create /dev/md0 --level=multipath --raid-devices=3 /dev/sda1 /dev/sdc1➥/dev/sdd1The kernel monitors the failure of the partition and activates a spare when it fails.However, the mdmpd daemon from the mdadm RPM package must be running to automaticallyadd a failed partition back to the array when it becomes available again.