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I'm attempting to get a software RAID array going using [this guide][1]. I've worked my way through preparing the second disk (/dev/sdb) without any errors.

After running:

mkswap /dev/md5

I run:

mdadm --examine --scan

Before piping it to my /etc/mdadm/mdadm.conf file. Problem is, I seem to be getting two results for the same device:

root@test:~# mdadm --examine --scan
ARRAY /dev/md/5 metadata=1.2 UUID=9985ef40:2eb8407d:c0a195b1:509136fe name=test:5
ARRAY /dev/md/1 metadata=1.2 UUID=34de79db:470dce04:ece45660:b4af82d5 name=test:1
ARRAY /dev/md/5 metadata=1.2 UUID=27796750:70770c0a:f5af0aca:5367090e name=test:5

I am following this tutorial. The machine I'm testing on had a software RAID array on it before, but I ran mdadm --zero-superblock on both partitions to ensure any remnants were gone.

Any ideas as to why I'm getting two entries for the same disk?

  • try mdadm --detail /dev/md* or --detail --scan – frostschutz Nov 23 '16 at 17:26
  • So those two commands list the last two entries in my original command. Is there a way to remove/delete the first one? Thanks again for your help on this. – tparrott Nov 23 '16 at 17:31
  • Hmmm, should be three. mdadm -v --examine --scan would list the devices. – frostschutz Nov 23 '16 at 17:39
  • Ok -- in looking at this, it appears as though the first /dev/md/5 device is pointing to /dev/sda5, which is probably my issue here. Any idea how I disassociate /dev/md/5 from /dev/sda5? – tparrott Nov 23 '16 at 17:41
  • If sda5 is not part of any raid, you can mdadm --zero-superblock it. – frostschutz Nov 23 '16 at 17:49
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/dev/md5 and /dev/md/5 is not the same thing. You probably actually do have three arrays, only you ended up with the same name for two of them.

/dev/md5 is a numbered md device, and /dev/md/5 is a named one. Names are arbitrary, can be anything. You could make a /dev/md/swap if you so desire. You update it with --update=name, on creation it's the --name= option, or simply specifying /dev/md/name in the first place. The manpage describes this in some detail.

Personally I don't like md names very much. It's particularly pointless when you end up using some number as a name. You can stick with numbers in the first place then. If you want names, use descriptive ones (root, home, swap, ...).

In general, mdadm --examine --scan is just a starting point. You really only need the UUID so you could do something like this:

ARRAY /dev/md0 UUID=9985ef40:2eb8407d:c0a195b1:509136fe
ARRAY /dev/md1 UUID=34de79db:470dce04:ece45660:b4af82d5
ARRAY /dev/md2 UUID=27796750:70770c0a:f5af0aca:5367090e

Don't forget to add a MAILADDR (and have a mdadm monitor service running).

  • Thanks for the reply -- how can you tell which one is the numbered and which is the named? They're both listed as /dev/md/5 in my mdadm output... – tparrott Nov 23 '16 at 17:18

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