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I have a 4-bay Synology with a 4x1TB RAID-5 volume that has been up and running without issues for 3-4 years. Of the slightly less than 3TB capacity about half is used.

Now, because RAID-5 doesn't really cut it, and 4 disks take 4 times as long to spin up and be checked when the disk station powers up in the morning (it turns on automatically on schedule at 7), plus noise, plus power consumption, and whatnot...

... after finding a still-sealed 3TB disk that I had bought half a year ago and forgotten about in the desk drawer, I've impulsively bought two 4TB drives with the idea of replacing the 4-disk RAID-5 configuration with a 2-disk setup.
Two 4TB disks, that's double the capacity (not like I really need it, but still) and only half the number of drives. Great.

Now, the obvious problem is that preferrably, the transition should happen without

  • losing all (or, well any) data
  • having to copy 1.5TB worth of medium and small files over GbE network and back

...either one being a major nuisance. Sadly, I know only too well what ordeal the network copy would be because I have a second disk station that pulls backups of the most vital data (not all) once per week -- it takes forever.

So, the idea was to pull out one disk from the RAID, freeing one bay. That'll work ("disk failed") thanks to redundancy.
Then plug the spare 3TB disk in, copy all data over on the device, destroy the old volume, replace disks, create new volume, copy data back. Take out spare disk (or leave it, whatever).

That won't work, as far as I can tell. Plugging in the spare disk will make it part of the RAID (but using only 1/3 of its capacity).

Somehow, I'd need a way to either convert a 4-disk RAID into a 3-disk RAID (hardly possible?), or a way of telling the disk station that the new disk that I'm plugging in is not supposed to replace a failed disk, and is not supposed to be filled with parity.

Is there a way of achieving this? Basically I need something like an emergency copy with a one-disk-failed array (onto a disk, pretending I don't have one). Something the like?

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Actually, there was a good thread in here, about shrinking Synology volumes. You need to Reduce the filesystem size, logical volume size, array size. Resize the file system back and convert the spare disks.

I was doing this couple of years ago, and the resizing was perfectly completed. Hope it'll help.

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Alas, so I tried...

Shrinking the volumes as suggested in the other thread didn't work out. Merely shrinking the file system took 2 days.

I left out shrinking the volume group both because there was no volume group at all (just the raw md device) and because my system doesn't have lvreduce (for a reason I don't know) anyway.

Failing one disk (prerequisite to shrinking the array) was easy to go, modifying the array not so. The system didn't like performing any operation I told it to do. It seems like that didn't destroy data, but it didn't work either.

Luckily, I had thought of plugging in a USB-3 external harddisk adapter, sudoing and doing a cp -r --preserve=all /volume1/* /volumeUSB/ prior to making any kind of modification (took only 3 hours).

That allowed me to copy the whole directory back after waiting 4 1/2 days until the disk station had parity-checked the newly created array (power off, pull out all disks, plug in new disks, power on). That, again, took one cp command, and 3 hours.

Copying the volume's mount point on the shell like this almost works. All files are retained, but shares must be re-created in the preferences panel (this doesn't lose any data!).
A few, but funnily not all, subdirectories in two particular shares had permissions that made these subdirectories inaccessible via SMB (no idea why or how!). chown -R root.users followed by chmod 770 on the respective share fixed it.

SurveillanceStation retains all recordings, but claims that they have been deleted (the files are still accessible, and appear under "recordings" though they do not show in the timeline, nor are they accessible via SS).

PhotoStation as well as everything else "just works" as if nothing had ever happened.

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