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diff --git a/docs/MOUNT_REQUIREMENTS.md b/docs/MOUNT_REQUIREMENTS.md new file mode 100644 index 0000000..9ccbd08 --- /dev/null +++ b/docs/MOUNT_REQUIREMENTS.md @@ -0,0 +1,72 @@ +--- +title: Mount Requirements +category: Booting +layout: default +SPDX-License-Identifier: LGPL-2.1-or-later +--- + +# Mount Point Availability Requirements + +systemd makes various requirements on the time during boot where various parts +of the Linux file system hierarchy must be available and must be mounted. If +the file systems backing these mounts are located on external or remote media, +that require special drivers, infrastructure or networking to be set up, then +this implies that this functionality must be started and running at that point +already. + +Generally, there are three categories of requirements: + +1. 🌥️ *initrd*: File system mounts that must be established before the OS + transitions into the root file system. (i.e. that must be stablished from + the initrd before the initrd→host transition takes place.) + +2. 🌤️ *early*: File system mounts that must be established during early boot, + after the initrd→host transition took place, but before regular services are + started. (i.e. before `local-fs.target` is reached.) + +3. ☀️ *regular*: File system mounts that can be mounted at any time during the + boot process – but which specific, individual services might require to be + established at the point they are started. (i.e. these mounts are typically + ordered before `remote-fs.target`.) + +Of course, mounts that fall into category 3 can also be mounted during the +initrd or in early boot. And those from category 2 can also be mounted already +from the initrd. + +Here's a table with relevant mounts and to which category they belong: + +| *Mount* | *Category* | +|---------------|------------| +| `/` (root fs) | 1 | +| `/usr/` | 1 | +| `/etc/` | 1 | +| `/var/` | 2 | +| `/var/tmp/` | 2 | +| `/tmp/` | 2 | +| `/home/` | 3 | +| `/srv/` | 3 | +| XBOOTLDR | 3 | +| ESP | 3 | + +Or in other words: the root file system (obviously…), `/usr/` and `/etc/` (if +these are split off) must be mounted at the moment the initrd transitions into +the host. Then, `/var/` (with `/var/tmp/`) and `/tmp/` (if split off) must be +mounted, before the host reaches `local-fs.target` (and then `basic.target`), +after which any remaining mounts may be established. + +If mounts such as `/var/` are not mounted during early boot (or from the +initrd), and require some late boot service (for example a network manager +implementation) to operate this will likely result in cyclic ordering +dependencies, and will result in various forms of boot failures. + +If you intend to use network-backed mounts (NFS, SMB, iSCSI, NVME-TCP and +similar, including anything you add the `_netdev` pseudo mount option to) for +any of the mounts from category 1 or 2, make sure to use a network managing +implementation that is capable of running from the initrd/during early +boot. [`systemd-networkd(8)`](https://www.freedesktop.org/software/systemd/man/latest/systemd-networkd.html) +for example works well in such scenarios. + +Note that +[`systemd-homed.service(8)`](https://www.freedesktop.org/software/systemd/man/latest/systemd-homed.html) +(which is a regular service, i.e. runs after `basic.target`) requires `/home/` +to be mounted. |