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8e3fee33af
This reverts commit 5e8ff010a1.
This broke all the URLs, we can't have that. (And actually, we probably don't
_want_ to make the change either. It's nicer to have all the pages in one
directory, so one doesn't have to figure out to which collection the page
belongs.)
73 lines
3.2 KiB
Markdown
73 lines
3.2 KiB
Markdown
---
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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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