OverlayFS with SD card and LUKS encrypted MD RAID?

Software SW tweaks
1

Hi,
I ave Turris Omnia with two internal drives and 64GB SD card; currently I have created MD-RAID 1 on top of the two rather noisy SATA HDDs which is also encrypted using LUKS, so I am thinking of adding a 64GB SD card I have spare from my camera after being replaced with 128GB version.

What I’d like to achieve is a persistent storage that will create an persistent overlay over the LUKS encrypted MD-RAID so that the spinning disks will be used less often; I would like the overlay filesystem to be a mountpoint that will first store data (non-persistently) in a RAM (let’s say I allocate 512MB), then it will write the changes to the silent SD card and after that it will commit the made changes to the LUKS encrypted MD RAID, is there a combination of tools in Turris OS or Linux in general which I can use to achieve this easily?

2

“overlay” kind of filesystems need your explicit decision which subtrees are located on which devices, I believe. What you want sounds more like bcache (or bcachefs). However, I can’t see a kernel module for bcache provided on Omnia ATM. (I might be wrong there.)

3

I know the explicit subtree, the entire /mnt/int_drives_01/ mount point… I’ll check the bcache…

4

I meant explicit decision about moving between SD card and the rotating drive(s), in your case. BTW, RAM caching should be done by default, regardless of filesystem.

5

I’d lie the writes to be pushed to the HDD once SD card is full or upon graceful / ungraceful shutdown (reboot).