I bought Synology 918+ and two Corsair MP500 120GB NVMe SSD in 2017. For just home usage, I just use NAS as daily backup and bit-torrent upload. Very light usage.

Recently, DSM said the SSD cache crashed. It happened a few times before. Each time it's on the first day of some month. DSM will do a data scrubbing on that day by default. During that process, the system will raise an error about SSD cache. I thought it was a compatiblility issue because as soon as I reboot the system (sometimes it had to be forced) and repair the SSD cache, it went back to work flawlessly.

FYI, MP500 is not on the compatible list.

This time, it's different. I re-inserted that two SSD into M.2 slots and reboot, then start repairing. A few minutes after repairing finished, DSM said it got multiple I/O error on SSD cache. Then, every apps on Synology became unresponsive. I had to pull out the SSD and took them to a local dealer to find out whether it's a compatible issue or hardware failure. I ran Corsair SSD Toolbox under Windows and it turns out each SSD already have 10TB read and 190TB write. After discussing this situation with someone on Reddit, it turns out that problem does not lies in the SSD cache implementation but lies in the way I uses those SSDs. The system doesn't actually write 190TB to the SSD but the write amplification does.

So here's my advice.

  • Try upgrading the RAM first before you consider using a SSD cache to speed up the whole system.
  • Always consider using a compatible SSD from the official list. Enterprise-level SSD recommended, SLC is the best but it's the most expensive.
  • Read-only cache preferred. Since it will not increase any data loss risks and significantly more bang for buck.
  • In order to eliminated write amplification effect, Do not allocate entire size of the SSD for cache volume, leave at least 20% unused.

And that's what you should be aware of:

  • SSD cache will not take participant in the sequential writes and reads. Most of activities are sequential workloads.
  • You should determine the bottleneck in your scenario before considering a SSD cache. The purpose of SSD cache is to offload some of the random access activities, so SSD cache should be effective if you're a heavy VM user.
  • Pay attention to SMART data periodically. Life span ratio not accurate all the time.

Some advices credited to ssps.