I just purchased a Synology DS918+ and have installed four 8TB Seagate Ironwolf drives.

One reason I chose the DS918 was the potential for future expandability using the DX 517 which provides 5 additional bays. My thought was that when I needed the added storage I could move from my 4-drive RAID 5 configuration to a 9-drive RAID 6 configuration.

However, I just noticed that the promotional material for Seagate Ironwolf includes the phrase "up to 8 bays".

WD Red drives carry a similar statement: "built for single-bay to 8-bay NAS systems."

I don't know whether this will prevent me from a 9-drive RAID configuration. Is this a physical "enclosure" limit based on the vibration dampening? Or is it a firmware limit on optimizing RAID that won't work for a 9th drive? Or is there some other limitation?

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1 Answer

After a bit more research, I have determined that it is enclosure-based due to something called "rotational vibration" (RV).

Synology's blog states:

Closely nested drives operating at the same time inside a NAS can result in vibration, noise, and heat. In addition, hard drives usually have high spindle speeds (RPM) so that disks can rotate at a faster speed and read data faster. Low spindle speeds create less vibration and noise, but they become a hindrance to achieving better read/write performance. Vibration is especially undesirable in multi-bay or rack systems, for it hampers stability of hard drive operation and even leads to data loss.

Seagate documents describe AgileArray:

AgileArray delivers drive balancing by using dual-plane balance and rotational vibration (RV) sensors...

Similarly, WD documents describe Rotary Acceleration Feed Forward:

Hard drive performance is degraded when a hard drive is exposed to vibration induced by one or more of its neighbors in the chassis. This induced vibration shakes the head off a track that is currently being read from or written to, resulting in retries and serious performance consequences.

Accordingly, it appears having two separate enclosures (4-drive and 5-drive) won't approach this design specification.