RAID with Raspberry Pi

  • Hi,

    I have a problem with last build OMV_3_0_99_RaspberryPi_2_3_4.9.80.img
    I have a Raspberry 3 B+ and Raspberry 3 B with the same problem for both of them

    I did a fresh install into a USB Card and installed it.
    After, I have 3.0.79 version of OMV and kernel 4.9.80-v7. In the RAID menu, I found my two disks (/dev/sda and /dev/sdb).

    Then I waited for the update and reboot.
    After, I have 3.0.99 version of OMV and kernel 4.14.30-v7. And in the RAID menu, I don't find my two disks. It's empty !
    I can't create a RAID 1 with my two disks.

    Do you have a solution to resolve my problem?


  • The first question you should ask yourself is why you want to play RAID. Most probably the answer has something to do with data protection and then you're already wrong (RAID is not backup).

    RAID-1 only protects from a hard drive fail (happens not that often) and doesn't provide any more benefits. It's all about increased availability and reliability.

    If you are concerned about availability and reliability why do you choose then the most crappy platform possible? Every Raspberry Pis from a NAS point of view is just horribly unreliable and way too slow anyway.

    • Those things suffer from underpowering problems
    • The CPU has only one single USB2 connection to the outside
    • If you want to access USB storage you want to use UAS (USB Attached SCSI). Cheap SBC like an Orange Pi Zero for $7 can do UAS but Raspberries not (they're only capable of the old and slow BOT mode)
    • Every disks connected to a Pi is behind the same internal USB hub
    • Even the Ethernet interface is behind the internal USB hub
    • On RPi 3 B+ it's not just one internal USB hub but two cascaded ones (they managed to make things even worse)
    • All devices behind these internal USB hubs have not only to share bandwidth but add to USB bus contention problems
    • The internal USB hub is a so called SPoF (single point of failure -- RAID is all and only about eliminating those!!!)

    Adding to this: to access USB disks flawlessly you need a perfectly working USB-to-SATA bridge inside the drive enclosure (99% of USB disks connected to a Pi do not apply). RAID rebuild times are horrible since bottlenecked by the single USB2 connection and shared bus. The time a rebuild takes your array has no redundancy any more. Such a RAID rebuild is disk and bus stress. What if now the most prominent SPoF on these thingies fails? When the internal USB hub decides to fail with heavy RAID accesses your whole array is gone instantly.

    RAID with USB disks is already insane given the many problems that prevent reliable operation. USB RAID with all disks behind one USB2 hub is close to stupid. Do yourself a favour and don't try this. It's a useless waste of at least one disk and the setup will fail when needed anyway (RAID rebuild after a disk failure, maybe even before. Just search the forum or see here directly).

  • Thanks you very much for your explain. You have open my eyes for RAID via RPi. For me, RPI is not expensive. And the RAID it's just perfect for me for data protection (in addition to a backup).

    OMV it's just perfect for a home NAS. I use this :)

  • And the RAID it's just perfect for me for data protection

    Again: RAID does NOT provide any means of data protection. It's only about increased availability (one disk fails but operation can continue without manual intervention -- that's the idea, often it doesn't work that way since hardware is too crappy, users do not test appropriately and are then surprised that their whole array has gone with the first failing disk).

    Availability (business continuity) is all what RAID is about (leaving aside the performance aspects when running larger RAID arrays on server grade hardware and not on toys).

    If you do RAID for data protection then you do it wrong. If you want to play RAID on a Raspberry toy you're doing wrong too. The hardware is way too unreliable to be able to increase reliability or to provide higher availability.

    If you're fine with abysmal NAS performance and some reliability issues then a Raspberry Pi is fine as single disk NAS. But that's it. And for the low performance these things are way too expensive.

  • >Again: RAID does NOT provide any means of data protection.

    Mirroring and RAID-5 protect your data against hardware failure. If you had a single hard disk (with no backup) and this disk failed, then your data has gone. With mirroring, one drive can fail and you don't lose your data. Is that not data protection? Sure, it's not a backup but to say it doesn't give any form of data protection is IMO a bit misleading. RAID is about data protection, maintaining up time and improved performance.

    >If you're fine with abysmal NAS performance

    I appreciate that using a Raspberry Pi NAS in a business situation is probably out but what about in small business or home media server? I've ended up here because a friend was asking about cost effective NAS solutions and OpenMediaVault seems perfect. I've just done a test install in a virtual machine to play with the functionality. I happen to have two spare 1TB 2.5" disk drives and a 32GB memory card which we can re-use.

    A client of mine has a QNAP TS-869 Pro which comes with 1GB RAM and a dual core Intel Atom D2701. The Raspberry Pi 4B compares pretty well with this.

    >some reliability issues then a Raspberry Pi

    What reliability issues exist? This is all new to me so just learning about what's possible.

  • While this is a terrible idea with usb drives not just the RPi, if you really want to use raid on an RPi with OMV, all you have to do is create the array from the command line. You can do everything else from the web interface. Just don't complain here when you have issues with the array assembling. This is a much greater and likely risk potentially causing you to lose all data than a drive failing.

    RAID is about data protection, maintaining up time and improved performance.

    Doubtful you would see improved performance on usb drives sharing a usb hub. And raid is about availability not data protection. Backups are for data protection. If you are just going to mirror the drives, rsync'ing (or rsnapshot) them together is a much better option in my opinion. If you accidentally delete file(s), mirroring will instantly delete both copies where a scheduled rsync will give you time. Rsnapshot would give you multiple copies. Most home users don't need realtime mirroring.

    What reliability issues exist?

    There is no redundancy on the board. It is powered by a usb port. It costs $35 (yes, I know the 4GB rpi is $55). You shouldn't expect server grade reliability out of it.

    A client of mine has a QNAP TS-869 Pro which comes with 1GB RAM and a dual core Intel Atom D2701. The Raspberry Pi 4B compares pretty well with this.

    The QNAP is designed to be a NAS and has extensive development to make it works as NAS. An RPi is design to be a small desktop system with I/O to do fun things.

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