I'm honestly fine with SnapRAID not being real time backup since 95% of the files residing on these arrays are media. The only reason I haven't gone SnapRAID + MergerFS yet is no built in caching mechanism. All my UnRAID shares are cache enable so writes go directly to a RAID0 SSD cache pool. I also have some shares (Downloads for example) that reside only on cache. I don't want to lose this type of configuration.
I don't think you will have to.
the only thing is, unlike Unraid, which has this options built int and configurable via more or less intuitive GUI
you will have to figure out how to get there on your own.
possible example of a setup
an OMV setup with SnapRaid and MergerFS installed.
OMV has plugins you can install and use to setup both tools. (SnapRaid plugin and Unionfilesystem plugin that provides UI for MergerFS pool setup.)
OMV also has plugin and UI options to setup cronjobs so you can setup data moving script.
FYI, I use OMV as an example as I have been playing with it for a while and currently planning out a similar setup myself. I am also planning to run this as a VM under Proxmox VE.
OMV is debian based if you care.
so at the quick glance :
an OMV setup with SR+MFS plugins
1 - OS drive
2 - A data drives pool with SR and mergerfs
3- a cache drive or drive pool using SSDs.
by default OMV does not share OS drive, so if you plan on using SSD as OS drive either plan for small SSD or partition the drive beforehand as needed. the primary OS partition(s) are not shareable or usable in any easy way from OMV UI.
so, setup OMV, finish config and install all plugins.
setup Data Pool with SnapRaid and MergerFs.
you can use any FileSystem you want on the drives themselves. unfortunately if you used default FS in unraid is probably a raserFS and not supported so you will have to copy the data off them and reformat.
not sure what your preferences are, I use BTRFS for my data drives if I can.
now setup your cache drive/pool.
setup the cache share, this will be used by all apps that you want to use cache for.
setup data share.
build the scripts that will recreate your current unraid functions as needed.
i.e. a script that would monitor a folder or folders on the cache share for files and copy/move them to your data pool.
a script that will run snapraid sync/scrub (this actually is build in into OMV snapraid plugin and you can just configure and activate it.) FYI a SMART also is built in the OMV by default and will monitor your disks live.
I guess you only need the script that will monitor folders on cache drive and copy/move files appropriately.
you might actually be able to take the script from unraid and modify it to work for you.
the only true functionality you will lose is automatic cache-to-data folder linkage . as in if you create a new folder to use on data pool you will have to add it to the cache drive manually and add it to the monitoring script manually. unless you manage to write a script that would look at the folder name on cache drive and search for the same on data drive. not really well versed in linux scripting so cannot say if possible.