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