Multipathing Configuration issue waiting to happen

Quite some time ago, I came across a quite hard to find issue during a consulting engagement, which i find worth mentioning. A 2 node RAC cluster running on RHEL4 x86-64 was relocated to a different data center. Apart from making sure, that the switch ports and Fibre Channel Ports are available on the new location, there is not much to worry about.

After the relocation, on one node the multipathing configuration, implemented with dev-mapper-multipath would not work. The command “multipath -ll” would just not return any output. After more than an hour, we pinned the issue down to the error message:

# multipath -v 3
#
# all paths in cache :
#



path sdh not found in pathvec

When checking what device sdh was, we realized that this was a KVM device, plugged in by the sysadmins.

May 27 14:46:26 host1 kernel: Attached scsi removable disk sdh at scsi10, channel 0, id 0, lun 0
May 27 14:46:26 host1 kernel: Type: Direct-Access ANSI SCSI revision: 02
May 27 14:46:26 host1 kernel: Vendor: KVM Model: vmDisk Rev: 0.01
May 27 14:46:26 host1 kernel: scsi10 : SCSI emulation for USB Mass Storage devices
May 27 14:46:26 host1 kernel: sr1: scsi3-mmc drive: 0x/0x caddy
May 27 14:46:26 host1 kernel: Type: CD-ROM ANSI SCSI revision: 02
May 27 14:46:26 host1 kernel: Vendor: KVM Model: vmDisk-CD Rev: 0.01

BTW: What is a KVM device?
Wikipedia states: A KVM switch (with KVM being an abbreviation for Keyboard, Video or Visual Display Unit, Mouse) is a hardware device that allows a user to control multiple computers from a single keyboard, video monitor and mouse.

We then added the device sdh to the multipath blacklist section in /etc/multipath.conf, and the problem was solved:

devnode_blacklist {
devnode “^sdh$”
}

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