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2.4. Multipath Devices in Logical Volumes
After creating multipath devices, you can use the multipath device names just as you would use a physical device name when creating an LVM physical volume. For example, if /dev/mapper/mpatha is the name of a multipath device, the following command will mark /dev/mapper/mpatha as a physical volume.
# pvcreate /dev/mapper/mpatha
You can use the resulting LVM physical device when you create an LVM volume group just as you would use any other LVM physical device.
If you attempt to create an LVM physical volume on a whole device on which you have configured partitions, the pvcreate command will fail.
When you create an LVM logical volume that uses active/passive multipath arrays as the underlying physical devices, you should include filters in the lvm.conf to exclude the disks that underlie the multipath devices.
This is because if the array automatically changes the active path to the passive path when it receives I/O, multipath will failover and failback whenever LVM scans the passive path if these devices are not filtered. For active/passive arrays that require a command to make the passive path active, LVM prints a warning message when this occurs. To filter all SCSI devices in the LVM configuration file (lvm.conf), include the following filter in the devices section of the file.
filter = [ "r/block/", "r/disk/", "r/sd.*/", "a/.*/" ]
After updating /etc/lvm.conf, it's necessary to update the initrd so that this file will be copied there, where the filter matters the most, during boot. Perform:
update-initramfs -u -k all
Every time either /etc/lvm.conf or /etc/multipath.conf is updated, the initrd should be rebuilt to reflect these changes. This is imperative when blacklists and filters are necessary to maintain a stable storage configuration.