Handle compressed root filesystem in ISO mode

[firasuke]

Ok, consider the case the root filesystem was compressed using squashfs or erofs into a file called rootfs.erofs.

According to this, I think booster should check whether the root filesystem is compressed/squashed or not, if it is then an additional mount command should be run:

...
mount -t erofs -o ro /dev/some_loop_device /some_mount_directory
...

So in addition to checking for /dev/sr0 and mounting it, booster should also check whether another parameter is passed that specifies the location of the root filesystem on /dev/sr0 whether it was compressed or not (for example another root= as an argument for booster and not for cmdline)..

[anatol]

Booster needs to add erofs filesystem detection.

By the chance do you have exact instructions on how to create a bootable ISO with efofs & squashfs?

[firasuke]

First of all you need to have a root file system (you can grab one of alpine's mini root file system for convenience).

It comes in .tar.gz format, so you should extract that into a directory say rootfs, and then create an erofs out of it by running:

mkfs.erofs rootfs.erofs rootfs

Now going back to our ISO structure:

- boot/ (contains `vmlinuz`, `initramfs`, `grub/` or `isolinux/` files)
- fs/ (contains rootfs.erofs) (some distros call `fs/` as `live/`)

Now an initramfs should detect we are in ISO mode and attempt to mount /dev/sr0 as iso9660 which is what booster started doing as of #223, but that is only in the case the root file system was the root layout of /dev/sr0.

In our case here, the root file system is still in erofs format and simply mounting /dev/sr0 won't suffice, as booster now needs to do another mount for the erofs file system then switch_root into that or something similar:

losetup /dev/loop0 /run/iso/fs/rootfs (or location of rootfs.erofs)
mount -t erofs -o ro /dev/loop0 /mnt/fs

Loop device number (could be based on what loop device is available, and the location /mnt/fs could be in /run or whatever you see fit, as the above is just an example for the sake of explaining this).

Now having a read-only filesystem is not always a good idea, so we may need to have a tmpfs or overlayfs mounted to simulate read-write mode until system is shutdown.

Hope that clears things up a bit.

[firasuke]

Any updates to this? as I am delaying the ISO release process until this feature is added :)

[anatol]

I just merged erofs fs detection code to master.

To get the schema you described working booster needs to mount a loop device on a file. So you can specify root=/rootfs.erofs and booster will handle mounting it.

Currently booster can mount a device file only, but it will be doable to add support for loopback devices in case if root is a regular file.

Are there any initramfs that already implement erofs root filesystems? I want to check their implementation as well.

[firasuke]

To get the schema you described working booster needs to mount a loop device on a file. So you can specify root=/rootfs.erofs and booster will handle mounting it.

Exactly!

Currently booster can mount a device file only, but it will be doable to add support for loopback devices in case if root is a regular file.

Ok, cool.

Are there any initramfs that already implement erofs root filesystems? I want to check their implementation as well.

I believe only Arch's ISO scripts support EROFS. Alpine's initramfs generation utility (mkinitfs) supports SquashFS (which providing support for is almost identical to EROFS).

[firasuke]

We should also handle the case of providing read-write support when in ISO mode (via overlayfs), which is also mentioned in #88.

[firasuke]

A proposal for the whole ISO / OverlayFS / SquashFS / EROFS situation (Getting booster to support handling ISOs with read-write support):

• The following directories need to be created by booster beforehand:

/media/cdrom # Where /dev/sr0 will be mounted
/media/rootfs/lower # Where /dev/loop0 will be mounted (Overlayfs step 1)
/media/rootfs/upper # (Overlayfs step 2)
/media/rootfs/work # (Overlayfs step 3)
/new_root # Where chroot / switch_root will take place

• The following modules might be needed beforehand:

erofs,iso9660,loop,overlay

• /dev/sr0 should be mounted ro as type iso9660 on /media/cdrom (according to FHS 3.0):

mount -t iso9660 -o ro /dev/sr0 /media/cdrom

• losetup should be run (the location of the compressed rootfs should be controlled by the variable root= which has recently added to booster):

losetup /dev/loop0 /media/cdrom/boot/rootfs.erofs

• /dev/loop0 should be mounted ro as type erofs on /media/rootfs/lower:

mount -t erofs -o ro /dev/loop0 /media/rootfs/lower

• overlay should be mounted rw as type overlay on /new_root with the following parameters (lowerdir=/media/rootfs/lower,upperdir=/media/rootfs/upper,workdir=/media/rootfs/work):

mount -t overlay -o lowerdir=/media/rootfs/lower,upperdir=/media/rootfs/upper,workdir=/media/rootfs/work overlay /new_root

[firasuke]

Any updates to this?

[firasuke]

What's the current situation on this?

[CosmicFusion]

squashfs as rootfs with a writable overlay would be very nice to have