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I have a TS-453D NAS with 4x10TB HDD disks. I have reinstalled it from scratch - it never asks me to format the disks though - but when I created the Storage Pool as a RAID5 it started to sync "something" and is taking about 14~16h (disks are not so fast). Is that sync really necessary? If so why?

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It's not strictly required, but it's a good idea. Since you're using a ready-made RAID solution you should treat it as a black box and trust that it knows what it's doing.

This type of RAID is sandwiched between the physical storage layer and the filesystem layer. It turns multiple drives into what appears like a single large drive, but doesn't care or understand what's on it. In particular it doesn't understand the filesystem created on this large virtual drive and, as a consequence, can't tell apart used disk areas from unused ones. It must assume that all disk space if equally important and its integrity must be maintained.

In RAID5 the redundancy is implemented by mathematically entangling corresponding addresses on all (three or more) drives. This mathematical entanglement is designed in such a way that content on any drive can be derived from content on the remaining N-1 drives, so if one of the drives dies, its replacement can be restored from the remaining drives. Your NAS is currently rewriting data on all the drives to put them in this entangled state. As a positive side effect, it's also ensuring that all sectors are writable and error-free.

More sophisticated RAID solutions that are integrated with filesystems can skip this step because they understand which pieces of disks are unused and don't have to be kept consistent.

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