SSD (Solid State Drive) caching is a relatively new feature that’s now available in some Synology NAS models. As the name implies, it saves active files on the SSDs, which allows them to be accessed faster than they would on the regular spinning drives.
SSD Caching is only supported in some Synology models. As you may guess, it’s not available in the lower cost models. This does make sense for reasons beyond just trying to move you to the more expensive models. You really need four drive bays for this to add performance. Since this is a performance feature Synology only includes this in their plus line or in the units targeted to large businesses. So yes, while not the only reason it’s limited, it SSD caching is a feature intended to move you up to the performance models.
Synology has a white paper (pdf) that talks about their SSD Cache technology in more detail.
Synology can do either read caching or full read & write caching. Read caching can be done with one SSD while write caching requires two solid state drives. Write caching requires two drives because they will be set up to be mirrored. This way when one drive fails no data will be lost. If a drive does fail in the cache mirror then all write caching is stopped and data is immediately written so that no data is lost when the other drive fails.
Caching is done for an entire volume so if you plan on doing SSD caching you should plan ahead by splitting your data across different volumes. Decide which data will benefit from caching and which will not. There is a 1TB limit to the size of the SSD cache. In addition, each 1 GB of SSD cache uses approximately 416KB of RAM so you’ll need to be sure you have enough RAM. When setting up the cache you’ll be told how much RAM will be used. The SSD Advisor will also warn you if you don’t have enough RAM for it’s recommended cache size.
As an example, I have two 120 GB Intel SSDs as a read-write cache. DSM says they can handle a 111 GB Cache and will use 45.1 MB of memory.
In my case I put my active data files on Volume 1. Volume 2 has my music, videos and older files that I’m simply storing. I also picked Volume 1 for all my package installations, even if they use data on other volumes. In other words archiving and entertainment aren’t cached. I use Synology for my Time Machine backups and those aren’t cached either. Even if caching did help my backups occur in the background so speed isn’t critical and not worth the expense.
In most situations files being streamed will not benefit from caching. The exception to this may be a file being streamed to multiple people or multiples times throughout the day.
I already mentioned the SSD Cache Advisor that Synology has. You’ll want to run this after you’ve split your data. Even then I got a recommendation that was higher than I was willing to spend on drives. A closer look at the numbers showed a usage spike on one day so I felt comfortable using a smaller cache. I do include some video as “data” when it’s part of my work plus I began a new backup of files on that volume so this would explain the spike on that day.
Finally, if you’re using a read/write cache then you must use a Uninterruptible Power Supply (UPS) so that when power is lost you will not lose data. You really should already be using a UPS but using a write cache without one is like playing Russian roulette with every chamber loaded.
Storage Manager can provide the raw statistics on how effective your cache is, but even in the warm-up phase, where little is already cached, I found Photo Station to be more responsive.