Most of them are outdated, but provide historical design context.
They are not user documentation and should not be treated as such.
Documentation is available here.
Node RPM Persistence After Upgrades
oVirt Node NG is an advancement over vintage Node, especially that it allows users to install RPMs or customize the system through configuration management tools. Some of these changes are migrated over from /etc after upgrades, and some are kept persistenly in /var, but any packages which are installed on a running layer will be lost after upgrades and need re-installation. This presents some problems with vendor tooling, IPMI, utilities to collect statistics, and other add-ons.
This feature presents a mechanism by which packages installed through yum/dnf can be saved and automatically re-applied when the OS is updated.
A deep dive presentation of the feature is available on youtube
- Name: Ryan Barry
- Email: firstname.lastname@example.org
- Status: On QA
- Last updated date: 29th Jan 2017
Benefit to oVirt
This feature will greatly improve the user experience of Node by maintaining a consistent userland and system configuration after upgrades. This allows for Node customizations to “stick”.
None! We simply need to add an additional yum plugin to imgbased.
Node NG performs a number of actions when updates are applied.
- A new layer is added
- An event fires for the new layer
- The new squashfs is extracted onto that layer
/rootare rsynced to the new layer
- UIDs and GIDs are synchronized across packages, in case a drift occurred inbetween images for UIDs/GIDs which are not fixed
- A new bootloader entry is added and set as the default
- An additional plugin can be added to imgbased which also consumes the ‘on-layer-added’ hook
A reliable mechanism must be written in order to capture packages as they are installed, however.
yum.PackageObject is able to immediately provide the location of a cached RPM (after downloading or from localinstall), and this is available as part of yum plugin hooks.
It may be possible to extend RPM, but some research must be done in order to discover whether RPM’s plugin interface is as informative as yum.PackageObject. It is possible that RPM’s plugin mechanism only triggers during the execution of the RPM specfile, and cannot be used to pin down the location of the actual package. This also provides the additional burden of maintaining a small codebase in C, though that may be an acceptable tradeoff.
yum provides a variety of places to add hooks, and actually somewhat more than what’s listed:
args clean close compare_providers config exclude historybegin historyend init postconfig postdownload postreposetup postresolve posttrans postverifytrans predownload prelistenabledrepos prereposetup preresolve pretrans preverifytrans verify_package
While not all of these hooks run while yum has the package cached, we can rely on
pretrans to have the cached package available, both for
yum localinstall and
Though the hook should be limited to only run during certain transaction types (install, upgrade), the transaction package object is available, and we can grab the local file from YumAvailablePackage.localPkg and save it off to /var, which is a persistent filesystem on Node NG, which is not versioned per layer.
The yum plugin will add a hook into
- The pretrans conduit is able to retrieve the transaction set from yum
- Inside this transaction set, if a package is being installed or updated,
TransactionSet.installedwill contain all package objects
- We can iterate over the package objects, and retrieve
TransactionSet.PackageObject.localpkg()(this is actually
po.localpkg, since yum uses short variable internally
- The package will be copied over to
- Using the yum cache is not reliable, as
yum clean allwould remove cached RPMs, and
yum localinstallnever touches the cache
- If the package set is empty, the plugin should check whether
uninstallhave been used as verbs
- If they are, a
posttranshook can be used to remove the specified packages from
/var/imgbased/persisted-rpms, so they are not spuriously installed again when users upgrade
The yum plugin should provide a configuration file, with only one parameter necessary:
Any package which is on the list of exclusions will not be persisted.
A new imgbased plugin will be added, which triggers when a new layer is added.
- This plugin will mount the new layer, and bind mount /var into the tree
- Then we’ll set up a local yum repository, pointed at
/var/imgbased/persisted_packages, and install all RPMs
With the packages saved in /var, imgbased.osupdater can set up a local yum repository which points at
/var/imgbased/persisted_packages (pathname to be determined), and install them when osupdater triggers during a Node update.