Comment 0 for bug 1538775

Release: Ubuntu 14.04.3 LTS
Kernel: linux-image-3.16.0-59-generic
A clean system installed fresh today (2016-01-27)

In attempting to configure a system to boot-from-SAN and enable multipath support I ran into an issue whereby despite the multiple paths being detected (when running the multipath command from the CLI) the configuration wasn't being enabled at boot. After examining /usr/share/initramfs-tools/scripts/local-top/multipath I found the following:

verbose && log_begin_msg "Waiting for scsi storage"
{ rmmod scsi_wait_scan ; modprobe scsi_wait_scan ; rmmod scsi_wait_scan ; } >/dev/null 2>&1
verbose && log_end_msg

The problem appears to be that the scsi_wait_scan module doesn't exist and so there is no wait before the multipath scan is performed. I managed to observe this briefly during bootup (with the script edited) and could see it performed the scan before sda/sdb was discovered.

I also found a debian bug report indicating the module was removed a while back (https://lists.debian.org/debian-kernel/2012/05/msg00791.html).

After adding in an artificial delay for testing the multipath command does what is expected and configures the paths accordingly. I'm not sure what the correct approach is if the scsi_wait_scan module is removed.

I also found that the same local-top script doesnt have the dm-round-robin module loaded (but it is included in the initrd by the associated hook script), however I'm not sure if that is by design. I know I need it for my specific use-case, but don't know if it is deliberately excluded to prevent breakage on SAN units that don't support native round-robin.