readonly upper branch and read/write lower branch

[IdrisMiles]

Hello, I'm trying to understand if MergerFS is suitable for my use case, my initial testing has proved unsuccessful but I may not be setting all the appropriate configuration to achieve my goal - or MergerFS is not intended for my use case.

The use case:
I have 2 identical filesystems acting in a master/replica setup, where one filesystem is read-only but fast to read from, the other is read-write but slow to read from.
My goal is to merge these 2 filesystems so that all reads are directed to the fast RO filesystem and writes are directed to the slow RW filesystem.

I've attempted the following:

mkdir readonly readwrite merged
echo "hello" > readonly/file.txt
echo "hello" > readwrite/file.txt

mergerfs -o allow_other,use_ino /tmp/readonly=RO:/tmp/readwrite=RW /tmp/merged

echo "world" >> merged/file.txt

cat merged/file.txt
# hello
# world

cat readonly/file.txt
# hello
# world

cat readwrite/file.txt
# hello

Firstly, this seems odd to me as the =RO filter does not seem to be respected?
Secondly, my desired results would actually be:

cat merged/file.txt
# hello

cat readonly/file.txt
# hello

cat readwrite/file.txt
# hello
# world

It may seem counter-intuitive to want the merged path to show the old contents but another process is in charge of syncing the changes to the readonly filesystem, so they would eventually become available.

Is what I'm hoping to achieve possible with MergerFS?

I've had a brief look at the policies and tried some variations but to no avail, all of the below produce the same results:

mergerfs -o allow_other,use_ino,category.search=ff,category.create=ff,category.action=ff /tmp/readonly=RO:/tmp/readwrite=RW /tmp/merged

mergerfs -o allow_other,use_ino,category.search=ff,category.create=all,category.action=all /tmp/readonly=RO:/tmp/readwrite=RW /tmp/merged

mergerfs -o allow_other,use_ino,category.search=ff,category.create=newest,category.action=newest /tmp/readonly=RO:/tmp/readwrite=RW /tmp/merged

mergerfs -o allow_other,use_ino,category.search=ff,category.create=epff,category.action=epff /tmp/readonly=RO:/tmp/readwrite=RW /tmp/merged

Some system details:
mergerfs version: 2.33.5
fusermount version: 2.9.2
CentOS Linux release 7.9.2009 (Core)

[trapexit]

https://github.com/trapexit/mergerfs#how-it-works

mergerfs does NOT support the copy-on-write (CoW) or whiteout behaviors found in aufs and overlayfs. You can not mount a read-only filesystem and write to it.

[trapexit]

https://github.com/trapexit/mergerfs#does-mergerfs-support-cow--copy-on-write--writes-to-read-only-filesystems

[IdrisMiles]

Ok so it's not suitable - thank you.

[trapexit]

No. You should use overlayfs.