73 lines
2.6 KiB
Markdown
73 lines
2.6 KiB
Markdown
---
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nav_order: 6
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---
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# Architecture of apply-live
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{: .no_toc }
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1. TOC
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{:toc}
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## Copying into an "underlay"
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As noted in the architecture doc, everything in rpm-ostree is oriented
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around creating and managing hardlinked complete bootable filesystem trees.
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In this flow then, `rpm-ostree install --apply-live strace` will first
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create a new pending deployment, run sanity tests on it, prepare it to be booted, etc.
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However, the first time `apply-live` is invoked, we create an `overlayfs`
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mount over `/usr`. It's mounted `ro` from the perspective of the rest
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of the system, but rpm-ostree can write to it.
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## Package and filesystem diffs
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When `apply-live` is invoked, rpm-ostree computes the diff between
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the source and target OSTree commit for `/usr`. If this is the *first* `apply-live`,
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the source commit is the booted commit. For subsequent invocations,
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it will be based on the current live commit.
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We also compute a package-level diff; this is how `apply-live`
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currently distinguishes between pure package additions versus upgrades.
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## Copying data for /usr
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Per the core OSTree model, almost everything we care about is in `/usr`.
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So the first step is to apply the diff to the transient writable `overlayfs`.
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One downside is that that this diff will take extra memory and disk space
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proportional to its size.
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## Updating /etc
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The second aspect we need to take care of is `/etc`. Normally, the libostree
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core handles the `/etc` merge during shutdown as part of `ostree-finalize-staged.service`,
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but we need to do it now in order to ensure that we get new config files
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(or remove ones).
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Note that the changes in `/etc` are persistent, live-applied changes there are
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also hence not updated transactionally. It is hence possible for configuration
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files to "leak" from partially applied live updates.
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## Updating /var
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Normally, libostree core never touches `/var`. Today rpm-ostree generates
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`systemd-tmpfiles` snippets for RPM packages which contain directories in
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`/var`. In a regular update, these will hence be generated at boot
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time by `systemd-tmpfiles-setup.service`.
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But here, we need to do this live. So rpm-ostree directly starts a
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transient systemd unit running `systemd-tmpfiles`.
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## Tracking live state
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Because the `overlayfs` is transient (goes away on reboot), the `apply-live`
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operation also writes its state into the transient `/run` directory, specifically
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a stamp file is stored at `/run/ostree/deployment-state/$deployid/`.
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Currently, there is also a persistent ostree ref `rpmostree/live-apply` for
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the current live commit. Eventually the goal is that libostree itself would
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gain direct awareness of live apply, and we wouldn't write a persistent ref.
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