Updating RAID1 to RAID5 / adding a third drive

  • Noob here.

    I'd like to grow my RAID1 into a RAID5 by adding another hard drive, but the user interface won't allow me to do it. The "Grow" button is greyed out. It won't even let me add it as a hot spare.

    I get this error:

    Failed to execute command 'export PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin; export LANG=C; mdadm --manage '/dev/md0' --add /dev/sde 2>&1' with exit code '1': mdadm: /dev/md0 has failed so using --add cannot work and might destroy mdadm: data on /dev/sde. You should stop the array and re-assemble it.

    Any ideas?

  • This might seem like a dumb question, but again, I'm a noob when it comes to this stuff.

    How exactly do you get the command line for mdadm to even launch.

    Also, I found this:

    • Do not use RAID arrays in production with drives connected via USB, neither hubs or different ports. This includes low power devices that do not have a SATA controller, e.g. Raspberry Pi, Pogoplugs and any low entry ARM SBC.

    I have a Raspberry Pi 3 as my server and drives connected through a powered USB hub. No good?

  • - You can't grow a RAID 1 (where the two drives are identical) into a RAID 5 (where data is striped over the drives). This would require rearranging data blocks among the 3 drives, which is not feasible.

    - A Raspberry Pi 3 is pretty much the worse platform possible for a NAS.

    - RAID over USB, especially slow USB like on a Raspberry Pi, is not recommended.

    - You get the command line by connecting to the NAS over SSH (with PuTTy or the web console plugin)

    My opinion would be to forget about RAID (why do you need it anyway ?) and set up a proper backup strategy instead.

  • This is all great information. Thank you. I suppose, for my needs, backup is more important than redundancy. I do have another drive that uses Rsync to back up to the RAID1. That's my little network share, and it handles that perfectly well for me. Much more reliable than my old Pogoplug was.

    Again, all of this is connected via a powered USB hub to the Raspberry Pi.

    I suppose I was misled a bit by stuff like this:

    Shouldn't I at least be able to mount the third drive to my RAID1 as a spare though?

    Would a RAID10 be a better solution with four drives? Wouldn't that give me backup AND redundancy?

  • The problem with the Raspberry is that it is superslow. Its network port and all 4 USB ports use the same internal port, that has limited speed. A NAS typically reads and writes data over the network, while reading or writing to the disk. With RAID1, it needs to write to two drives simultaneously. With RAID5, it will be using all 3 USB ports, plus the network, at the same time over the same saturated bus.

  • RAID over USB = A bad idea. Will it work? The answer is maybe or even yes, for a time. There are known problems with setting up RAID arrays over USB. You'll be risking losing everything.

    Instead of using RAID1, you could Rsync your two disks and get real backup, reliably.

    You could use a hub and multiple disks with an R-PI, if you can tolerate the performance, but creating RAID arrays with them is an "at your own risk" proposition.

    There's a few ways to get to the command line.

    Two of them are connect a monitor and keyboard connected to your R-PI, or install PuTTY on a Windows Client. To use PuTTY, as detailed in this guide on page 27, you might need to permit root logins over SSH. The following pages will show you how to use PuTTY to log into your R-PI.

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