In 15.10, dracut and initramfs-tools cannot coexist which makes it hard to regenerate the initramfs.
root@builder:/# apt-get install dracut initramfs-tools
Reading package lists... Done
Building dependency tree
Reading state information... Done
initramfs-tools is already the newest version.
initramfs-tools set to manually installed.
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
dracut : Conflicts: initramfs-tools but 0.120ubuntu6 is to be installed
initramfs-tools : Conflicts: linux-initramfs-tool
E: Unable to correct problems, you have held broken packages.
From https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1109029:
"Just in case someone is interested: I've built an ppa that allows installing draucut next to initramfs:
https://launchpad.net/~morty/+archive/ubuntu/dracut-release
And yes, they do conflict as they both want to build the init.rd. And initramfs-tools wins, because i comes after d. I fixed that using dpkg-divert."
Status changed to 'Confirmed' because the bug affects multiple users.