My unRAID Array is setup as follows.

2TB - Parity

2TB - Storage

2TB - Storage

2TB - Storage

1TB - Storage

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So I have 7TB of total storage. What size cache drive should I install?

 

Thank in advance for help on this.

 

The Goat

Completely depends on your use. The way I see it there are 3 ways to use cache.

 

1) Caching writes to user shares. This was the original use for cache. If you set a user share to use cache then any writes to the user share will go to the cache drive first, then that data is moved to the array at the scheduled time. Writing cache is faster than writing the parity array since parity isn't involved. But since parity isn't involved, anything written isn't protected until it is moved to the array. The default schedule moves cached user share writes once per day, but you can set the schedule to make it more (or less) frequent if you want. If you want to use your cache this way, you need to size it so it can hold as much as you expect to write before the next scheduled move, plus however much space you need for other uses of cache (see below). I personally don't use my cache this way since I don't care about write speed.

 

2) Storing application data. VMs, dockers, and plugins often need to read and write data used by the specific applications. Putting that data on a cache-only share can improve performance of the applications since parity isn't involved in writing the data, and will also keep from spinning up the parity drive to write this data. This is one of the main ways I use cache, for my dockers.

 

3) If you have an SSD for cache then there can be some advantage to keeping some user shares there since SSDs don't spin and may use less power. I have Pictures and Music cache-only user shares I keep copies of some of my pictures and music on to allow screensavers, wallpaper changers, and music applications on my network quick access to this data.

 

So, it depends on what you want to do with cache.

What it looks like from the options listed I am really wanting to add it for the second reason application/plugin/docker caching. This server is primarily used for PLEX and for file  backup/sharing. But those files aren't accessed that rapidly so they wouldn't really need to be cached first then moved. So the main cache would really just be used for PLEX caching. So with that being said if I have 7TB of possible data storage for PLEX what would be a decent size of a SSD cahce drive?

I noticed you went back the array size.

Cache size really has little to do with array size, as trurl alluded to.

 

So for example, using trurl's 3 categories, a certain user might need

[*]About 200GB as write cache, based on a worst-case-scenario estimate of maximum daily write

[*]3 VM's (32GB vdisk each) + some dockers => about 120GB

[*]An 80GB music collection => need about 100GB for future expandability.

Hence, a cache disk of about 500GB would suffice (adding about 100GB for temporary files e.g. downloads in progress etc.)

Notice I did not need to know how big the above user's array is.

 

Hence I suggest you work through the list as trurl presented and give yourself an estimate. If in doubt, err on the side of over-estimation.

Thanks for all the help. Much appreciated.