mirror of
https://github.com/systemd/systemd.git
synced 2024-11-01 17:51:22 +03:00
ff2c2d0850
@bertob wants us to be strict here, and only have one "#" header per markdown file, and use "##" (or "###", …) for all others. Interestingly, we mostly got this right already, but this fixes a few cases where this wasn't correct.
210 lines
10 KiB
Markdown
210 lines
10 KiB
Markdown
---
|
|
title: Automatic Boot Assessment
|
|
category: Booting
|
|
layout: default
|
|
---
|
|
|
|
# Automatic Boot Assessment
|
|
|
|
systemd provides support for automatically reverting back to the previous
|
|
version of the OS or kernel in case the system consistently fails to boot. This
|
|
support is built into various of its components. When used together these
|
|
components provide a complete solution on UEFI systems, built as add-on to the
|
|
[Boot Loader
|
|
Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION). However, the
|
|
different components may also be used independently, and in combination with
|
|
other software, to implement similar schemes, for example with other boot
|
|
loaders or for non-UEFI systems. Here's a brief overview of the complete set of
|
|
components:
|
|
|
|
* The
|
|
[`systemd-boot(7)`](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)
|
|
boot loader optionally maintains a per-boot-loader-entry counter that is
|
|
decreased by one on each attempt to boot the entry, prioritizing entries that
|
|
have non-zero counters over those which already reached a counter of zero
|
|
when choosing the entry to boot.
|
|
|
|
* The
|
|
[`systemd-bless-boot.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-bless-boot.service.html)
|
|
service automatically marks a boot loader entry, for which boot counting as
|
|
mentioned above is enabled, as "good" when a boot has been determined to be
|
|
successful, thus turning off boot counting for it.
|
|
|
|
* The
|
|
[`systemd-bless-boot-generator(8)`](https://www.freedesktop.org/software/systemd/man/systemd-bless-boot-generator.html)
|
|
generator automatically pulls in `systemd-bless-boot.service` when use of
|
|
`systemd-boot` with boot counting enabled is detected.
|
|
|
|
* The
|
|
[`systemd-boot-check-no-failures.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-boot-check-no-failures.service.html)
|
|
service is a simple health check tool that determines whether the boot
|
|
completed successfully. When enabled it becomes an indirect dependency of
|
|
`systemd-bless-boot.service` (by means of `boot-complete.target`, see
|
|
below), ensuring that the boot will not be considered successful if there are
|
|
any failed services.
|
|
|
|
* The `boot-complete.target` target unit (see
|
|
[`systemd.special(7)`](https://www.freedesktop.org/software/systemd/man/systemd.special.html))
|
|
serves as a generic extension point both for units that shall be considered
|
|
necessary to consider a boot successful on one side (example:
|
|
`systemd-boot-check-no-failures.service` as described above), and units that
|
|
want to act only if the boot is successful on the other (example:
|
|
`systemd-bless-boot.service` as described above).
|
|
|
|
* The
|
|
[`kernel-install(8)`](https://www.freedesktop.org/software/systemd/man/kernel-install.html)
|
|
script can optionally create boot loader entries that carry an initial boot
|
|
counter (the initial counter is configurable in `/etc/kernel/tries`).
|
|
|
|
## Details
|
|
|
|
The boot counting data `systemd-boot` and `systemd-bless-boot.service`
|
|
manage is stored in the name of the boot loader entries. If a boot loader entry
|
|
file name contains `+` followed by one or two numbers (if two numbers, then
|
|
those need to be separated by `-`) right before the `.conf` suffix, then boot
|
|
counting is enabled for it. The first number is the "tries left" counter
|
|
encoding how many attempts to boot this entry shall still be made. The second
|
|
number is the "tries done" counter, encoding how many failed attempts to boot
|
|
it have already been made. Each time a boot loader entry marked this way is
|
|
booted the first counter is decreased by one, and the second one increased by
|
|
one. (If the second counter is missing, then it is assumed to be equivalent to
|
|
zero.) If the "tries left" counter is above zero the entry is still considered
|
|
for booting (the entry's state is considered to be "indeterminate"), as soon as
|
|
it reached zero the entry is not tried anymore (entry state "bad"). If the boot
|
|
attempt completed successfully the entry's counters are removed from the name
|
|
(entry state "good"), thus turning off boot counting for the future.
|
|
|
|
## Walkthrough
|
|
|
|
Here's an example walkthrough of how this all fits together.
|
|
|
|
1. The user runs `echo 3 > /etc/kernel/tries` to enable boot counting.
|
|
|
|
2. A new kernel is installed. `kernel-install` is used to generate a new boot
|
|
loader entry file for it. Let's say the version string for the new kernel is
|
|
`4.14.11-300.fc27.x86_64`, a new boot loader entry
|
|
`/boot/loader/entries/4.14.11-300.fc27.x86_64+3.conf` is hence created.
|
|
|
|
3. The system is booted for the first time after the new kernel is
|
|
installed. The boot loader now sees the `+3` counter in the entry file
|
|
name. It hence renames the file to `4.14.11-300.fc27.x86_64+2-1.conf`
|
|
indicating that at this point one attempt has started and thus only one less
|
|
is left. After the rename completed the entry is booted as usual.
|
|
|
|
4. Let's say this attempt to boot fails. On the following boot the boot loader
|
|
will hence see the `+2-1` tag in the name, and hence rename the entry file to
|
|
`4.14.11-300.fc27.x86_64+1-2.conf`, and boot it.
|
|
|
|
5. Let's say the boot fails again. On the subsequent boot the loader hence will
|
|
see the `+1-2` tag, and rename the file to
|
|
`4.14.11-300.fc27.x86_64+0-3.conf` and boot it.
|
|
|
|
6. If this boot also fails, on the next boot the boot loader will see the
|
|
tag `+0-3`, i.e. the counter reached zero. At this point the entry will be
|
|
considered "bad", and ordered to the end of the list of entries. The next
|
|
newest boot entry is now tried, i.e. the system automatically reverted back
|
|
to an earlier version.
|
|
|
|
The above describes the walkthrough when the selected boot entry continuously
|
|
fails. Let's have a look at an alternative ending to this walkthrough. In this
|
|
scenario the first 4 steps are the same as above:
|
|
|
|
1. *as above*
|
|
|
|
2. *as above*
|
|
|
|
3. *as above*
|
|
|
|
4. *as above*
|
|
|
|
5. Let's say the second boot succeeds. The kernel initializes properly, systemd
|
|
is started and invokes all generators.
|
|
|
|
6. One of the generators started is `systemd-bless-boot-generator` which
|
|
detects that boot counting is used. It hence pulls
|
|
`systemd-bless-boot.service` into the initial transaction.
|
|
|
|
7. `systemd-bless-boot.service` is ordered after and `Requires=` the generic
|
|
`boot-complete.target` unit. This unit is hence also pulled into the initial
|
|
transaction.
|
|
|
|
8. The `boot-complete.target` unit is ordered after and pulls in various units
|
|
that are required to succeed for the boot process to be considered
|
|
successful. One such unit is `systemd-boot-check-no-failures.service`.
|
|
|
|
9. `systemd-boot-check-no-failures.service` is run after all its own
|
|
dependencies completed, and assesses that the boot completed
|
|
successfully. It hence exits cleanly.
|
|
|
|
10. This allows `boot-complete.target` to be reached. This signifies to the
|
|
system that this boot attempt shall be considered successful.
|
|
|
|
11. Which in turn permits `systemd-bless-boot.service` to run. It now
|
|
determines which boot loader entry file was used to boot the system, and
|
|
renames it dropping the counter tag. Thus
|
|
`4.14.11-300.fc27.x86_64+1-2.conf` is renamed to
|
|
`4.14.11-300.fc27.x86_64.conf`. From this moment boot counting is turned
|
|
off.
|
|
|
|
12. On the following boot (and all subsequent boots after that) the entry is
|
|
now seen with boot counting turned off, no further renaming takes place.
|
|
|
|
## How to adapt this scheme to other setups
|
|
|
|
Of the stack described above many components may be replaced or augmented. Here
|
|
are a couple of recommendations.
|
|
|
|
1. To support alternative boot loaders in place of `systemd-boot` two scenarios
|
|
are recommended:
|
|
|
|
a. Boot loaders already implementing the Boot Loader Specification can simply
|
|
implement an equivalent file rename based logic, and thus integrate fully
|
|
with the rest of the stack.
|
|
|
|
b. Boot loaders that want to implement boot counting and store the counters
|
|
elsewhere can provide their own replacements for
|
|
`systemd-bless-boot.service` and `systemd-bless-boot-generator`, but should
|
|
continue to use `boot-complete.target` and thus support any services
|
|
ordered before that.
|
|
|
|
2. To support additional components that shall succeed before the boot is
|
|
considered successful, simply place them in units (if they aren't already)
|
|
and order them before the generic `boot-complete.target` target unit,
|
|
combined with `Requires=` dependencies from the target, so that the target
|
|
cannot be reached when any of the units fail. You may add any number of
|
|
units like this, and only if they all succeed the boot entry is marked as
|
|
good. Note that the target unit shall pull in these boot checking units, not
|
|
the other way around.
|
|
|
|
3. To support additional components that shall only run on boot success, simply
|
|
wrap them in a unit and order them after `boot-complete.target`, pulling it
|
|
in.
|
|
|
|
## FAQ
|
|
|
|
1. *Why do you use file renames to store the counter? Why not a regular file?*
|
|
— Mainly two reasons: it's relatively likely that renames can be implemented
|
|
atomically even in simpler file systems, while writing to file contents has
|
|
a much bigger chance to be result in incomplete or corrupt data, as renaming
|
|
generally avoids allocating or releasing data blocks. Moreover it has the
|
|
benefit that the boot count metadata is directly attached to the boot loader
|
|
entry file, and thus the lifecycle of the metadata and the entry itself are
|
|
bound together. This means no additional clean-up needs to take place to
|
|
drop the boot loader counting information for an entry when it is removed.
|
|
|
|
2. *Why not use EFI variables for storing the boot counter?* — The memory chips
|
|
used to back the persistent EFI variables are generally not of the highest
|
|
quality, hence shouldn't be written to more than necessary. This means we
|
|
can't really use it for changes made regularly during boot, but can use it
|
|
only for seldom made configuration changes.
|
|
|
|
3. *I have a service which — when it fails — should immediately cause a
|
|
reboot. How does that fit in with the above?* — Well, that's orthogonal to
|
|
the above, please use `FailureAction=` in the unit file for this.
|
|
|
|
4. *Under some condition I want to mark the current boot loader entry as bad
|
|
right-away, so that it never is tried again, how do I do that?* — You may
|
|
invoke `/usr/lib/systemd/systemd-bless-boot bad` at any time to mark the
|
|
current boot loader entry as "bad" right-away so that it isn't tried again
|
|
on later boots.
|