why OMV thinks that a perfectly valid RAID array with a valid file system on it is not an acceptable device (whereas it used to allow for it in prior versions).
Why?
Note that I/O in RAID5 requires that all disks be available, because data and parity are stripped over a minimum of three drives.
The problem with USB and multiple drives is "bandwidth contention". USB is "serial" - I.E. only one drive at a time can access the serial bus at any given instant. SATA and SAS are "parallel" meaning that it is possible for all drives to access the PCI bus at the same time. (Or at least it seems that way because the PCI bus is blazingly fast, relative to drive I/O.) When an array is assembling, on boot up, drives must be "available" when queried. With USB, instant availability is not guaranteed so assembling the array may fail.
A PCI bus is faster than the collection of drives in RAID5. This is NOT the case with USB.
If you absolutely insist on RAID5, use an external drive enclosure that supports hardware RAID5. They present the array as a single drive to the OS, with no issues caused by the USB connection. (But they have their own issues in that they're a single point of failure and, with hardware RAID, drives may be married to the controller.)
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So what does the above mean? Users were creating RAID arrays, over USB, that may work for "awhile". Then the RAID5 array breaks; it won't assemble, it won't recover when a drive fails, etc. and they come to the forum for tech support. What can be done? Nothing. They've lost it all.
The best thing to do is to simply "not allow it". If you want RAID5, use something with SATA or SAS ports.
For SBC's:
- If you want RAID1, Rsync will work fine for that purpose. Rsync is actually better than RAID1 in that an Rsync'ed clone provides backup. (RAID1 does not.)
- If you want to aggregate a bunch of disks (JBOD) use the MergerFS plugin.
- If you want to be able to recover a failed drive, look at the SNAPRAID plugin.
All of the above techniques work well with SBC's. RAID5 doesn't.