When Ignition is fetching a resource over http(s), if the resource is unavailable Ignition will continually retry to fetch the resource with an exponential backoff between requests.
For a given retry attempt, Ignition will wait 10 seconds for the server to send the response headers for the request. If response headers are not received in this time, or an HTTP 5XX error code is received, the request is cancelled, Ignition waits for the backoff, and a new request is made.
Any HTTP response code less than 500 results in the request being completed, and either the resource will be fetched or Ignition will fail.
Ignition will initially wait 100 milliseconds between failed attempts, and the amount of time to wait doubles for each failed attempt until it reaches 5 seconds.
Ignition has support for fetching files over the S3 protocol. When Ignition is running in Amazon EC2, it supports using the IAM role given to the EC2 instance to fetch protected assets from S3. If IAM credentials are not successfully fetched, Ignition will attempt to fetch the file with no credentials.
When a Container Linux machine first boots, it's possible that an earlier installation or other process has already provisioned the disks. The Ignition config can specify the intended filesystem for a given device, and there are three possibilities when Ignition runs:
- There is no preexisting filesystem.
- There is a preexisting filesystem of the correct type, label, or UUID (e.g. the Ignition config says
/dev/sdashould beext4, and it isext4). - There is a preexisting filesystem of an incorrect type, label, or UUID (e.g. the Ignition config says
/dev/sdashould beext4, and it isbtrfs).
In the first case, when there is no preexisting filesystem, Ignition will always create the desired filesystem.
In the second two cases, where there is a preexisting filesystem, Ignition's behavior is controlled by the wipeFilesystem flag in the filesystem section.
If wipeFilesystem is set to true, Ignition will always wipe any preexisting filesystem and create the desired filesystem. Note this will result in any data on the old filesystem being lost.
If wipeFilesystem is set to false, Ignition will then attempt to reuse the existing filesystem. If the filesystem is of the correct type, has a matching label, and has a matching UUID, then Ignition will reuse the filesystem. If the label or UUID is not set in the Ignition config, they don't need to match for Ignition to reuse the filesystem. Any preexisting data will be left on the device and will be available to the installation. If the preexisting filesystem is not of the correct type, then Ignition will fail, and the machine will fail to boot.
When resolving paths, Ignition follows symlinks on all but the last element of a path. This ensures existing symlinks on a filesystem can be overwritten while still following symlinks as expected. When writing files, links, or directories, Ignition does not allow following symlinks outside the specified filesystem. When writing files, links, or directories on the root filesystem, Ignition follows symlinks as if it were executing in that root; a symlink to /etc is followed to /etc on the root filesystem. When writing files, links, or directories to any other filesystem, Ignition fails if it tries to follow a symlink outside that filesystem.
Ignition fully supports distributions which have SELinux enabled. It requires that the distribution ships the setfiles utility. The kernel must be at least v5.5 or alternatively have this patch backported.
The wipePartitionEntry and shouldExist flags control what Ignition will do when it encounters an existing partition. wipePartitionEntry specifies whether Ignition is permitted to delete partition entries in the partition table. shouldExist specifies whether a partition with that number should exist or not (it is invalid to specify a partition should not exist and specify its attributes, such as size or label).
The following table shows the possible combinations of whether or not a partition with the specified number is present, shouldExist, and wipePartitionEntry, and the action Ignition will take:
| Partition present | shouldExist | wipePartitionEntry | Action Ignition takes |
|---|---|---|---|
| false | false | false | Do nothing |
| false | false | true | Do nothing |
| false | true | false | Create specified partition |
| false | true | true | Create specified partition |
| true | false | false | Fail |
| true | false | true | Delete existing partition |
| true | true | false | Check if existing partition matches the specified one, fail if it does not |
| true | true | true | Check if existing partition matches the specified one, delete existing partition and create specified partition if it does not match |
A partition matches if all of the specified attributes (label, start, size, uuid, and typeGuid) are the same. Specifying uuid or typeGuid as an empty string is the same as not specifying them. When 0 is specified for start or size, Ignition checks if the existing partition's start / size match what they would be if all of the partitions specified were to be deleted (if allowed by wipePartitionEntry), then recreated if shouldExist is true.
Specifying number as 0 will use the next available partition number. Partition number 0 is disallowed on disks with partitions that specify shouldExist as false. If number is not specified it will be treated as 0.
Specifying start as 0 will use the starting sector of the largest available block. This is not necessarily the first available block large enough.
If start is not specified and a partition with the same number exists, Ignition will use the start of the existing partition, unless wipePartitionEntry is set.
If start is not specified and there is no existing partition, or wipePartitionEntry is set, Ignition will use the starting sector of the largest block, as if start were set to 0.
Specifying size as 0 means the partition should span to the end of the largest available block. If the starting sector is not within the largest available block, Ignition will fail.
If size is not specified and a partition with the same number exists, it will use the value of the existing partition, unless wipePartitionEntry is set.
If size is not specified and there is no existing partition, or wipePartitionEntry is set, size act as if it were set to 0 and use the size of the largest block.
Ignition supports fetching and merging multiple configs. This replaces the append functionality of the Ignition 2.x.0 specification. There are several rules that determine how configs get merged. When a child config is merged with a parent, generally the child config's values override the parent config's values.
If a parent and child object are being merged, the fields in the child object take precedence over the fields in the parent config. If a field in the child object is not specified, the field from the parent is used instead.
All lists of objects have a field that uniquely identifies that object. If a child config contains an entry that matches an entry already specified in the parent config, those entries are merged. A few sections of the config are exempt from this behavior. See the configuration specification for a complete listing. Generally the only lists that are simply appended are those that specify arguments to commands like mkfs or mdadm.
Since files, directories, and links all describe filesystem entries can conflict, these lists are deduplicated across each other. This means a file in a child config can replace a link in the parent, or a directory in a child config can replace a file in the parent.
A child config can specify children of its own. Those children are merged into their parent config before that config is merged into its own parent. If a config specifies multiple children, those children are merged in the order they appear.