Does the *physical* order/location of drives in a mdadm-managed RAID-10 array matter

raid10software-raid

i've setup a 4-drive RAID-10 array using mdadm-managed, software-raid on an x86_64 box.

it'd up & running and works as expected,

cat /proc/mdstat
    md127 : active raid10 sdc2[2] sdd2[3] sda2[0] sdb2[1]
          1951397888 blocks super 1.2 512K chunks 2 far-copies [4/4] [UUUU]
          bitmap: 9/466 pages [36KB], 2048KB chunk

atm the four SATA drives are physically plugged into the motherboard's 1st four SATA ports.

i'd like to gather the necessary/complete info for catastrophic recovery. reading starting here, http://neil.brown.name/blog, and the mailing list, i'm not yet completely confident i have it right.

i understand 'drive order matters'. is that logical, &/or physical order that matters?

if i unplugged the four drives in this array, and plugged them each back into different ports on the motherboard or a pci card, as long as i've changed nothing in software config, will the array correctly auto-re-assemble?

Best Answer

As @Fox said, the metadata will guarantee that the array can be assembled no matter what order the drives are detected in.

You should of course be thinking about the devices by their serial number or some other property unique to the physical device, as opposed to the device node name. e.g.:

$ ls -la /dev/disk/by-id/ata-ST3320418AS_6VM9PNFT
lrwxrwxrwx 1 root root 9 2011-11-15 23:20 /dev/disk/by-id/ata-ST3320418AS_6VM9PNFT -> ../../sde

because which physical devices get assigned which device nodes on boot isn't guaranteed. That could be important if you ever need to remove a device, etc. So in my case I think about that disk as serial 6VM9PNFT, not /dev/sde.

As for backing up the metadata, I don't think it's that important because as long as your array assembles you'll have the metadata. If the array doesn't assemble then what is the point of the metadata? Really you aren't meant to be messing around with metadata, you're supposed to be keeping enough available devices!

However, if you really want to back it up, you can dump it out by executing mdadm -E /dev/sde1 for each of the member devices in each of your arrays.

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