Rescuing Linux

Sometimes you need perform irregular, unpleasant tasks involving a Linux box. One generally picks up the techniques via experience. This kind of thing tends to become less necessary as one learns Linux better, and thus it's easy to lose currency. The introduction of systemd, for instance, changed almost all of this shit up.

initramfs

Almost all distributions ship kernels making use of initramfs these days. An initramfs can be embedded directly into the kernel image (see kernel config entry CONFIG_INITRAMFS_SOURCE), but it is usually shipped as its own file instead, and specified by the bootloader. An initramfs is a (possibly compressed) cpio archive. On boot, it is unpacked into a tmpfs. The compelling advantage of initramfs is the ability to mount the true root filesystem (which might be on NFS, or encrypted, etc.) from userspace, with a minimal filesystem such as userspace expects.

The initramfs-tools-core package ships lsinitramfs and unmkinitramfs to easily list or extract the contents of an initramfs file.

You're unlikely to run into initrd these days, but it can be unpacked the same way (the difference is in how it's mounted during boot). Initramfs on Debian are named initrd-*.

If built into the kernel, there are no extra considerations for the initramfs. If it's a distinct file, it needs to live somewhere visible to the bootloader. If booting directly from UEFI, it needs live in the ESP. So long as it's kept in the same directory as the kernel image, you ought be fine. Be sure to copy the initramfs along with the kernel image if you're ever backing up the kernel, or moving it to another machine, etc.

External links

• Debian wiki page for initramfs
• Kernel documentation for Ramfs, Rootfs, and Initramfs