Rebuilding an initramfs: dracut -f versus update-initramfs -u, and verifying the result

Goal

Regenerate the initramfs for the currently running (or a specified) kernel after a change that affects early boot — a new storage driver, a LUKS configuration change, an mdadm array change, or an /etc/fstab//etc/crypttab edit that early userspace needs to know about — and confirm the new image actually contains what changed.

Prerequisites

Root access; enough free space in /boot for a new image alongside the old one; the currently booted kernel still working (do this before rebooting, not while diagnosing a boot that is already broken, unless working from a chroot).

Steps

1. Copy the current image first (cp -p /boot/initramfs-<version>.img /root/; on Debian/Ubuntu /boot/initrd.img-<version>) so it can be copied back if the new one fails. On RHEL, Fedora and SUSE (dracut-based systems), rebuild for the running kernel with dracut -f (-f/--force overwrites the existing initramfs file); for any other kernel name image and version explicitly, e.g. dracut -f /boot/initramfs-<version>.img <version> (SUSE names it /boot/initrd-<version>). Inside a chroot, uname -r reports the rescue kernel, so always pass the version there.
2. On Debian, Ubuntu and derivatives (initramfs-tools), rebuild the existing image with update-initramfs -u -k <version> (-u "updates an existing initramfs"; without -k it targets the newest installed kernel, not necessarily the running one; use -k all to rebuild for every installed kernel after a change that affects all of them, such as adding a filesystem module or changing /etc/crypttab).
3. Know when a rebuild is required versus a mere update-grub: adding/removing an out-of-tree storage driver, changing which modules dracut.conf/initramfs-tools includes, changing a LUKS keyfile or crypttab entry, or changing the RAID layout mdadm must assemble at boot. A kernel package upgrade normally triggers this automatically via the package manager's hooks; a manual configuration change usually does not.
4. Verify contents before rebooting: lsinitrd lists the contents of a dracut-built initramfs image (for the current kernel by default, or a given image path); on Debian-family systems, lsinitramfs /boot/initrd.img-<version> lists the archive's files. Grep the listing for the module or file you expect (a driver .ko, a crypttab entry, an mdadm.conf) to confirm it is present.
5. Keep at least one older kernel and its matching initramfs installed as a fallback entry in the boot menu; do not remove the previous kernel package until the new initramfs has been booted successfully at least once.
6. Reboot to test. If the system fails to boot, select the fallback kernel entry at the boot menu — this is why step 5 matters — and rebuild again after diagnosing the problem from a working session.

Expected result

The new image's listing (lsinitrd / lsinitramfs) shows the driver, crypttab entry or mdadm configuration that prompted the rebuild; the next boot reaches the root filesystem without dropping to an initramfs shell.

Limits and test basis

Neither dracut -f nor update-initramfs -u has an undo command; the safety nets are the copied image (copy it back from the fallback kernel or a rescue boot) and the fallback kernel entry. Regenerating for the wrong kernel version (a common mistake when several are installed) silently leaves the booted kernel's own image unchanged.