Synology SSD cache size requirement

Question

I've been running a new Synology RS3621RPxs for several weeks, and my 4TB SSD cache shows:

Cache composition: Reusable (341 GB) / Total (3.6 TB)

If I'm reading that correctly, my nas isn't even using 10% of the available SSD cache. According to the Synology website this was the right size for my array, but it seems like overkilL! My cache hit rate is around 85%

The NAS is used primarily as an NFS data store for other hosts using ESXi. (So large VMDK files primarily). This SSD is a RO cache (single SSD), and I'm thinking of switching to a pair of NVMe sticks for RW cache. But, what size should I buy? Does RW cache increase demands? I hate to spend the money on 2x4TB of cache if most will go unused!

Answer 1

How much of your cache you can use fully depends on how you're accessing data. If you're literally only ever reading 10 MB 85 % of times, and the rest of the accesses is totally random and uncorrelated, well you'll only be able to make productive use of 10 MB in your cache.

Point is that making your cache makes it more likely that a latter access still hits something that is already cached. "For several weeks" doesn't sound very long in this context; unless you have a single fixed application with a definite data set size, it's likely that your access patterns (e.g. the database tables accessed?) shift over time and you get to use more of your cache.

But: low absolute used cache volume at high cache hit rate is the optimum use case for cache – so I really wouldn't complain!