What is "squashfs" image?


I want to reset my Archer C2 to the default settings. On the Backup Flash/Firmware page, there is an option "Perform Reset", along with it is noted:

To reset the firmware to its initial state, click "Perform reset" (only possible with squashfs images).

How can I tell whether my system is using squashfs image or not?

I'm running the OpenWrt 19.07.1 image mentioned at https://openwrt.org/toh/tp-link/archer_c2_ac750

This is the download link: (https://github.com/orangepizza/openwrt/releases/download/19.07.1/ArcherC2V1_tp_recovery.bin);

I haven't tried the perform reset button becuase I'm afraid of corrupting the setup and having to re-flash the firmware. So, is it safe to do? And what would happen if I tried and I don't have squashfs image?

This is not the original OpenWrt image.
The original OpenWrt image is squashfs.

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@drsly Just grepping dmesg is sufficient to know whether you are using a squashfs image (and like trendy points out, it's also in the file name of the official OpenWrt images).

On my router e.g.:

root@OpenWrt:~# dmesg|grep "squashfs filesystem"
[    2.353771] VFS: Mounted root (squashfs filesystem) readonly on device 31:5.

Basically, as you can see here, your rootfs is read-only and there is a writeable /overlay partition where OpenWrt settings (and packages you installed e.g.) are stored. A reset will wipe/reformat the overlay. That's all there's to it.

The first time you boot OpenWrt, it will use the hardcoded device settings in the read-only rootfs to configure your device, saving those settings on /overlay. It will do the same after a reset.

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