Filter disks that underlie the Multipath devices.
 

"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. T his 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."

I don't quite understand this. Why should we filter this since we have a blacklist in multipath.conf file and filter what?

Can anyone help me understand this?

And also how to configure multipath to active/passive or active/active mode? what is the default mode?


Thanks in advance.

When you have multipathing to a dual-controller RAID you will have two low-level SCSI devices, say /dev/sdc and /dev/sdd that are actually two paths to the same device. multipathd will create /dev/dm-0 on top of these two. The multipath.conf blacklist only prevents multipath from looking at certain drives.

Sending a read to /dev/sdc might cause it to be the active path on the RAID. If you don't have a filter, LVM might scan /dev/sdd causing the RAID to fail over and your performance to get terrible. The filter is to prevent lvmdiskscan from looking at the sd drives underlying /dev/dm-0.

That said, I don't think you need to write a filter. You just need to set multipath_component_detection correctly in /etc/lvm/lvm.conf. There's also a check for software RAID components md_component_detection.

Hi,

Thanks! :)

Br,

Hi,

What about the second question?
how to configure multipath to active/passive or active/active mode? what is the default mode?

Thanks