I was just introduced to multipathing in our production environment and had never heard of the concept prior. After some digging I think I'm starting to get a handle on how the concept works in theory but I'm having some trouble extrapolating that to what I'm seeing on the box I'm working on.
From multipath -ll I get output like:
mpath0 (36000d3100088060000000000000000b9) dm-0 COMPELNT,Compellent Vol size=60G features='1 queue_if_no_path' hwhandler='0' wp=rw `-+- policy='round-robin 0' prio=1 status=active |- 0:0:0:0 sda 8:0 active ready running |- 0:0:1:0 sdd 8:48 active ready running |- 1:0:0:0 sdi 8:128 active ready running `- 1:0:1:0 sdl 8:176 active ready running
From fdisk -l I know that those are all 60GB disks, with the same partition setup:
Disk /dev/sda: 64.4 GB, 64424509440 bytes 255 heads, 63 sectors/track, 7832 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Device Boot Start End Blocks Id System /dev/sda1 * 1 13 104391 83 Linux /dev/sda2 14 7832 62806117+ 8e Linux LVM
What is confusing to me though is how the partitions are actually mounted on the server:
$ df -h Filesystem Size Used Avail Use% Mounted on /dev/mapper/VolGroup00-LogVol00 30G 26G 3.8G 87% / /dev/mapper/mpath0p1 99M 49M 46M 52% /boot tmpfs 16G 232M 16G 2% /dev/shm /dev/mapper/mpath2p1 493G 226G 242G 49% /u02
Just considering /boot for now: It is mounted to mpath0p1, I can see that much. But how does this correspond to the physical disk/LVM behind the multipath?