Not with any builtin tools. Unraid is built on top of md, so it is a block device. As result, it can have a caching layer put in front of it easier than something FUSE based like MergerFS.
This doesn't mean that a crafty person couldn't write a mechanism to periodically sweep completed files from one location to another though[emoji1]
I do not think, unraid is build on top of md. it uses a modified md code but one that have been rewritten by LimeTech as a custom module that functions similar to what snapraid+ a union file system like mergerfs does, but with additional benefits of being real time parity raid-like setup.
I have run unRaid server for 3+ years before moving off it.
unraid has a caching mechanism built in into the system. if you have the drive(s) you just turn on the cache in settings and point the system to the drives be used for cache.
other than that it is function just like snapraid+mergerFS (or any other drive pooling system) except with a real time protection of the data.
here is a few key points
1.
UnRaid -- a single pool of mixed drives to be shared by the system
MergerFS(or other similar) -- same
2.
UnRaid -- a single pool of mixed drives protected via Parity Raid-Like setup where you can pull individual drives (except parity drive) and read data on them in any other system capable of mounting them.
yet the data is protected via Parity mechanism like a real raid 5/6 automatically and on the fly.
in essence UnRaid server works like SnapRaid+MergerFS(or similar) +a real time data validation and protection mimicking real raid setup.
SnapRaid -- same as unraid above but not real time. you can pool the drives of different size into a raid like setup where data is protected using Parity mechanism but the actual checks and balances are done on schedule intervals rather than real time as the data is written to pool.
SnapRaid is also transparent to the user as each protected drive can be used independently not like a raid setup where devices is pooled together into single virtual device.
3.
Unraid -- offers all of the benefits of real raid system described above plus ability to use drives of different sizes, array expansion on the fly and ability to use write cache for speed, although when using cache drives
data is not protected until it is moved to the data pool.
yes cache will speed up your writes but if something happens to cache drive in mid use your data is gone.
most common, default setup in UnRaid is to run cache sync scripts on schedule rather real time, so until script is run, your data is in limbo.