mount SD read-only and use overlay ramfs #70

fsagbuya · 2025-01-09T16:07:29+08:00

fsagbuya commented

2025-01-09 16:07:29 +08:00

Description

mount SD read-only by default and use overlay ramfs
Add option to easily mount SD r/w for development

Boot log:

[    2.160993] EXT4-fs (mmcblk0p2): mounted filesystem 44444444-4444-4444-8888-888888888888 ro with ordered data mode. Quota mode: disabled.

Runtime:

bash-5.2# df -h
Filesystem      Size  Used Avail Use% Mounted on
overlay         1.0G   48K  1.0G   1% /
devtmpfs        469M     0  469M   0% /dev
tmpfs           487M  4.0K  487M   1% /run
tmpfs           487M     0  487M   0% /dev/shm

Closes #69

### Description - mount SD read-only by default and use overlay ramfs - Add option to easily mount SD r/w for development Boot log: ``` [ 2.160993] EXT4-fs (mmcblk0p2): mounted filesystem 44444444-4444-4444-8888-888888888888 ro with ordered data mode. Quota mode: disabled. ``` Runtime: ``` bash-5.2# df -h Filesystem Size Used Avail Use% Mounted on overlay 1.0G 48K 1.0G 1% / devtmpfs 469M 0 469M 0% /dev tmpfs 487M 4.0K 487M 1% /run tmpfs 487M 0 487M 0% /dev/shm ``` Closes #69

fsagbuya added 1 commit 2025-01-09 16:07:30 +08:00

mount SD read-only and use overlay ramfs 52dfbaeec5

sb10q commented

2025-01-09 17:41:06 +08:00

Add option to easily mount SD r/w for development

If I understand correctly you currently have to replace the entire SD image to use that option. Doesn't sound very easy.

Would be easier to manually remount the SD card RW and modify some file to activate the option.

> Add option to easily mount SD r/w for development If I understand correctly you currently have to replace the entire SD image to use that option. Doesn't sound very easy. Would be easier to manually remount the SD card RW and modify some file to activate the option.

👍 1

fsagbuya commented

2025-01-14 13:33:08 +08:00

Upon testing, it seems manually remounting as RW in runtime is not possible in an overlayfs setup. Running mount -o remount,rw / fails because the root is mounted as an overlay type, combining a read-only SD card mount at /mnt.ro with a RAM-based overlay at /mnt.overlay. This actually makes sense for our goal of preventing SD card corruption, since all writes go to RAM while keeping the SD card safe.

Let me know if you had a different approach in mind.

Upon testing, it seems manually remounting as RW in runtime is not possible in an `overlayfs` setup. Running `mount -o remount,rw /` fails because the root is mounted as an overlay type, combining a read-only SD card mount at `/mnt.ro` with a RAM-based overlay at `/mnt.overlay`. This actually makes sense for our goal of preventing SD card corruption, since all writes go to RAM while keeping the SD card safe. Let me know if you had a different approach in mind.

sb10q commented

2025-01-15 10:39:40 +08:00

There's always the option of having a third partition with configuration. Or just use the FAT boot partition for that.

👍 1

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Reference: M-Labs/nix-servo#70