++blog: disk based booting
This commit is contained in:
Nico Schottelius
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title: Bye, bye netboot
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pub_date: 2021-08-31
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author: ungleich infrastructure team
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twitter_handle: ungleich
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_hidden: no
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_discoverable: yes
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abstract:
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Data Center Light servers are switching to disk based boot
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body:
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## Introduction
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Since the very beginning of the [Data Center Light
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project](/u/projects/data-center-light) our servers have been
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*somewhat stateless* and booted from their operating system from the
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network.
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From today on this changes and our servers are switched to boot from
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an disk (SSD/NVMe/HDD). While this first seems counter intuitive with
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growing a data center, let us explain why this makes sense for us.
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## Netboot in a nutshell
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There are different variants of how to netboot a server. In either
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case, the server loads an executable from the network, typically via
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TFTP or HTTP and then hands over execution to it.
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The first option is to load the kernel and then later switch to an NFS
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based filesystem. If the filesystem is read write, you usually need
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one location per server or you mount it read only and possibly apply
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an overlay for runtime configuration.
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The second option is to load the kernel and an initramfs into memory
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and stay inside the initramfs. The advantage of this approach is that
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no NFS server is needed, but the whole operating system is inside the
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memory.
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The second option is what we used in Data Center Light for the last
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couple of years.
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## Netboot history at Data Center Light
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Originally all our servers started with IPv4 PXE based
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netboot. However as our data center is generally speaking IPv6 only,
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the IPv4 DHCP+TFTP combination is an extra maintenance and also a
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hindrance for network debugging: if you are in a single stack IPv6
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only network, things are much easier to debug. No need to look for two
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routing tables, no need to work around DHCP settings that might
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interfere with what one wants to achieve via IPv6.
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As the IPv4 addresses became more of a technical debt in our
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infrastructure, we started flashing our network cards with
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[ipxe](https://ipxe.org/), which allows even older network cards to
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boot in IPv6 only networks.
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Also in an IPv6 only netboot environment, it is easier to run
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active-active routers, as hosts are not assigned DHCP leases. They
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assign addresses themselves, which scales much nicer.
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## Migrating away from netbooting
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So why are we migrating away from netbooting, even after we migrated
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to IPv6 only networking? There are multiple aspects:
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On power failure, netbooted hosts lose their state. The operating
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system that is loaded is the same for every server and needs some
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configuration post-boot. We have solved this using
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[cdist](https://www.cdi.st/), however the authentication-trigger
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mechanism is non-trivial, if you want to keep your netboot images and
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build steps public.
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The second reason is state synchronisation: as we are having multiple
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boot servers, we need to maintain the same state on multiple
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machines. That is solvable via CI/CD pipelines, however the level of
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automation on build servers is rather low, because the amount of OS
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changes are low.
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The third and main point is our ongoing migration towards
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[kubernetes](https://kubernetes.io/). Originally our servers would
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boot up, get configured for providing ceph storage or to be a
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virtualisation host. The amount of binaries to keep in our in-memory
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image was tiny, in the best case around 150MB. With the migration
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towards kubernetes, every node is downloading the containers, which
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can be comparable huge (gigabytes of data). The additional pivot_root
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workarounds that are required for initramfs usage are just an
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additional minor point that made us question our current setup.
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## Automating disk based boot
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We have servers from a variety of brands and each of them comes with a
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variety of disk controllers: from simple pass-through SATA controllers
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to full fledged hardware raid with onboard cache and battery for
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protecting the cache - everything is in the mix.
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So it is not easily possible to generate a stack of disks somewhere
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and then add them, as the disk controller might add some (RAID0) meta
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data to it.
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To work around this problem, we insert the disk that is becoming the
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boot disk in the future into the netbooted servers, install the
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operating system from the running environment and at the next
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maintenance window ensure that the server is actually booting from it.
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If you are curious on how this works, you can checkout the script that
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we use for
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[Devuan/Debian](https://code.ungleich.ch/ungleich-public/ungleich-tools/-/blob/master/debian-devuan-install-on-disk.sh)
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and
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[Alpine Linux](https://code.ungleich.ch/ungleich-public/ungleich-tools/-/blob/master/alpine-install-on-disk.sh)
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## The road continues
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While a data center needs to be stable, it also needs to adapt to
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newer technologies or different flows. The disk based boot is our
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current solution for our path towards kubernetes migration, but who
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knows - in the future things might look different again.
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If you want to join the discussion, we have a
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[Hacking and Learning
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(#hacking-and-learning:ungleich.ch)](/u/projects/open-chat/) channel
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on Matrix for an open exchange.
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Oh and in case [you were wondering what we did
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today](https://twitter.com/ungleich/status/1432627966316584968), we
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switched to disk based booting ;-).
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