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storage: extend auto install documentation
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This document describes Agama's approach to configure storage using a profile for unattended
installation.

## Legacy AutoYaST Specification
## Agama and AutoYaST

The Agama profile can contain either a `storage` section or a `legacyAutoyastStorage` one.
AutoYaST profiles can be used with Agama offering a 100% backward compatibility.

The rest of this document describes the `storage` section.
The `legacyAutoyastStorage` section of the Agama profile is a 1:1 representation of the XML
specification of AutoYaST. No JSON validation will be performed for it.

That `legacyAutoyastStorage` is a 1:1 representation of the XML specification of AutoYaST. No
JSON validation will be performed for it.
~~~json
{
"legacyAutoyastStorage": [
{
"use": "all",
"partitions": []
}
]
}
~~~

### Implementation Considerations for AutoYaST Specification

Expand All @@ -27,7 +36,153 @@ specify the size of a partition or to determine the default list of subvolumes w
See also the sections "Automatic Partitioning" and "Guided Partitioning" at the AutoYaST
documentation for situations in which AutoYaST uses the standard YaST `GuidedProposal` as fallback.

## Basic Structure of the Storage Section
### Problems with the AutoYaST Schema

The AutoYaST schema is far from ideal and it presents some problems.

#### Everything Is a Drive or a Partition Section

This could seem a minor detail, but it has several implications:

* A `<type>` property is required to indicated the type of device (*RAID*, *LVM*, etc).
* Some properties could be meaningless for the selected type.
* Having a `<partitions>` section for describing logical volumes is weird.

~~~xml
<partitioning config:type="list">
<drive>
<type config:type="symbol">CT_LVM</type>
<disklabel>gpt</disklabel> <!-- It does not make sense for a volume group -->
<partitions config:type="list"> <!-- It really means logical volumes -->
<partition>
<partition_id>131</partition_id> <!-- It does not make sense for a logical volume -->
</partition>
</partitions>
</drive>
</partitioning>
~~~

#### Directly Formatting Devices is Hammered

A `<partitions>` section is still needed for directly formatting a device, which shows the abuse of
the schema.

~~~xml
<partitioning config:type="list">
<drive>
<disklabel>none</disklabel>
<partitions config:type="list">
<partition>
<filesystem config:type="symbol">btrfs</filesystem>
</partition>
</partitions>
</drive>
</partitioning>
~~~

#### Selecting Devices is Difficult and Limited

The AutoYaST schema allows selecting specific devices by using the `<skip_list>` property. This
forces to use inverse logic when looking for a device. For example, if you want to select a disk
bigger than 1 GiB, then you have to skip the smaller disks:

~~~xml
<partitioning config:type="list">
<drive>
<skip_list config:type="list">
<!-- skip devices that are smaller than 1GB -->
<listentry>
<skip_key>size_k</skip_key>
<skip_value>1048576</skip_value>
<skip_if_less_than config:type="boolean">true</skip_if_less_than>
</listentry>
</skip_list>
</drive>
</partitioning>
~~~

The partitions to remove are selected by means of the `<use>` property, which is very limited. It
only allows removing everything, nothing, specific partition numbers or linux partitions.

~~~xml
<partitioning config:type="list">
<drive>
<device>/dev/sdc</device>
<use>2</use> <!-- Removes the partition number 2 -->
<partitions config:type="list">
...
</partitions>
</drive>
</partitioning>
~~~

The property `<partition_nr>` is used for reusing a partition. And again, this option is very
limited, allowing selecting a partition only by its number.

~~~xml
<partitioning config:type="list">
<drive>
<device>/dev/sdc</device>
<partitions config:type="list">
<partition>
<partition_nr>1</partition_nr> <!-- Reuse the partition number 1 -->
</partition>
</partitions>
</drive>
</partitioning>
~~~

Note that you could indicate the same partition number for deleting (`<use>`) and for reusing (`<partition_nr>`).

#### Devices Are Created in a Indirect Way

For creating new LVM volume groups, RAIDS, etc, it is necessary to indicate what devices to use as
logical volumes or as RAID members. In AutoYaST, the partitions have to indicate the device they are
going to be used by.

~~~xml
<partitioning config:type="list">
<drive>
<device>/dev/sda</device>
<partitions config:type="list">
<partition>
<raid_name>/dev/md/0</raid_name> <!-- Indicate what device is going to use it -->
</partition>
</partitions>
</drive>
<drive>
<device>/dev/sdb</device>
<partitions config:type="list">
<partition>
<raid_name>/dev/md/0</raid_name>
</partition>
</partitions>
</drive>
<drive>
<device>/dev/md/0</device>
</drive>
</partitioning>
~~~

It would be more natural to indicate the used devices directly in the RAID or logical volume drive.

## The New Storage Schema

Agama offers its own storage schema which is more semantic, comprehensive and flexible than the
AutoYaST one.

The new schema allows:

* To clearly distinguish between different types of devices and their properties.
* To perform more advanced searches for disks, partitions, etc.
* To indicate deleting and resizing on demand.

The Agama schema is used by a new Agama specific proposal. This decouples the algorithm from the
AutoYaST one, making much easier to support new use cases and avoiding backward compatibility with
fringe AutoYaST scenarios. It also supports some features that are not available in the AutoYaST
proposal like deleting or resizing partitions on demand.

### Basic Structure of the Storage Section

A formal specification of the outer level of the `storage` section would look like this.

Expand Down Expand Up @@ -67,7 +222,7 @@ That allows to rely on the YaST component known as `GuidedProposal`. That altern
removed as soon as all the capabilities of that `GuidedProposal` could be expressed in terms of a
regular storage configuration like the one explained above.

## Entries for Describing the Devices
### Entries for Describing the Devices

The formal specification of the previous section can be extended as we dive into the structure.

Expand Down Expand Up @@ -234,7 +389,7 @@ it) to allocate two file systems.
}
```

## Specifying the Size of a Device
### Specifying the Size of a Device

When creating some kinds of devices or resizing existing ones (if possible) it may be necessary to
specify the desired size. As seen in the specification above, that can be done in several ways.
Expand All @@ -260,7 +415,7 @@ It is also possible to specify "current" as a size value for partitions and logi
already exist in the system. The usage of "current" and how it affects resizing the corresponding
devices is explained at a separate section below.

### Under Discussion
#### Under Discussion

As explained, it should be possible to specify the sizes as "default", as a range or as a fixed
value. But in the last two cases, a parseable string like "40 GiB" may not be the only option to
Expand All @@ -269,13 +424,13 @@ represent a size. The following two possibilities are also under consideration.
- `{ "gib": 40 }`
- `{ "value": 40, "units": "gib" }`

## Partitions Needed for Booting
### Partitions Needed for Booting

Using a `boot` entry makes it possible to configure whether (and where, using an alias) Agama
should calculate and create the extra partitions needed for booting. If the device is not
specified, Agama will take the location of the root file system as a reference.

## Searching Existing Devices
### Searching Existing Devices

Many sections in the profile are used to describe how some devices must be created, modified or even
deleted. In the last two cases, it's important to match the description with one or more devices
Expand Down Expand Up @@ -418,7 +573,7 @@ drive, it will be considered to contain the following one.
}
```

### Under Discussion
#### Under Discussion

Very often, `search` will be used to find a device by its name. In that case, the syntax could be
simplified to just contain the device name as string.
Expand Down Expand Up @@ -458,7 +613,7 @@ above, it would be possible to use the key as name of the property, resulting in
}
```

## Referencing Other Devices
### Referencing Other Devices

Sometimes is necessary to reference other devices as part of the specification of an LVM volume
group or RAID. Those can be existing system devices or devices that will be created as response to
Expand Down Expand Up @@ -533,7 +688,7 @@ to be a reference to all the devices. As a consequence, this two examples are eq
}
```

### Under Discussion
#### Under Discussion

In addition to aliases, a `search` section could be accepted in all the places in which an alias can
be used. In that case, the scope of the search would always be the whole set of devices in the
Expand All @@ -554,13 +709,13 @@ system (so the same conditions can be matched by a disk, a partition, an LVM dev
}
```

## Keeping an Existing File System or Encryption Layer
### Keeping an Existing File System or Encryption Layer

The entries for both `encryption` and `filesystem` contain a flag `reuse` with a default value of
false. It can be used in combination with `search` to specify the device must not be re-encrypted
or re-formatted.

## Deleting and Shrinking Existing Devices
### Deleting and Shrinking Existing Devices

The storage proposal must make possible to define what to do with existing partitions and logical
volumes. Even with existing MD RAIDs or LVM volume groups.
Expand Down Expand Up @@ -694,7 +849,7 @@ space first by shrinking the partitions and deleting them only if shrinking is n
}
```

## Using the Automatic Proposal
### Using the Automatic Proposal

On the first implementations, Agama can rely on the process known as Guided Proposal to calculate
all the needed partitions, LVM devices and file systems based on some general product settings and
Expand Down

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