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Expert Partitioner: advanced Btrfs features

The section "more features for the future" of the SLE-15-SP1 Partitioner Features document lists some Btrfs features to be added to the Partitioner. Quoting directly that document:

  • Multi-device Btrfs (a Btrfs filesystem can expand through several block devices, combining features of RAID and LVM in that regard).
  • Proper representation of the Btrfs subvolumes (e.g. as a nested list with properties instead of a plain list of paths) in a more discoverable place.

The present document tries to discuss how those goals can be achieved.

The target Btrfs features at a glance

First of all, let's take a sligthly deeper look to these two Btrfs features in order to understand their basis.

Multi-device filesystem

As already mentioned, this is not so different from the capabilities already offered by LVM or software RAIDs. It basically refers to the ability of Btrfs of distributing its data and metadata over several block devices. That can be done in several modes (for both the data and the metadata, that can be configured separately): single, dup, raid0, raid1, raid10, raid5 and raid6.

As mentioned in the corresponding documentation, a Btrfs filesystem can be directly created over one or several devices and more devices can be added at any point after the filesystem has been created.

Btrfs subvolumes

Subvolumes are a very special feature of Btrfs that is not present in any other Linux filesystem. Each Btrfs filesystem can contain several subvolumes, defined in the corresponding Btrfs documentation as "independently mountable POSIX filetree that are not a block device (and cannot be treated as one)".

Btrfs subvolumes can be nested inside each other. In fact, all Btrfs filesystems contain a top-level subvolume (with Btrfs id 5) containing all the files, directories and other subvolumes. Thus, each subvolume except the top-level one has a parent subvolume. In addition, many SUSE products define an extra subvolume named "@" as an extra top-level container. Last but not least, filesystems snapshots are also implemented as subvolumes nested into an special subvolume called ".snapshots". As a result, the structure of subvolumes of a SUSE Btrfs filesystem can look like this.

\-- @
    +-- @/home
    +-- @/var
    |    +-- @/var/cache
    |    \-- @/var/lib/libvirt
    +-- @/tmp
    \-- @/.snapshots
         +-- @/.snapshots/1/snapshot
         \-- @/.snapshots/2/snapshot

Each subvolume can have its own configuration in many aspects. That includes its mount options (every subvolume is somehow seen by the system as a filesystem on its own), its Copy on Write (CoW) operation mode, its usage quotas and (in extreme cases) even its own RAID mode.

Last but not least, a given set of subvolumes can become "shadowed" if a separate filesystem is mounted in a way that makes them not longer relevant. For example, imagine a root Btrfs filesystem with the subvolumes /home, /var/lib and /var/cache. If a separate partition is then formatted and mounted in /var, two of the subvolumes will become irrelevant because they cannot be accessed.

Traditional Partitioner UI

Before coming with a solution to support the new features in the upcoming versions of the Partitioner, let's discuss the limitations in that regard of the current Partitioner, as it is shipped in openSUSE Leap 15.1 and SLE-15-SP1.

Filesystems coupled to their block devices

Since filesystem traditionally just sit on top of an existing block device, they have been so far represented just as a property of that underlying block device. Like the column "FS Type" in the following screenshot, that represents the filesystems being used in the sda1, sda2, sda3 and sda4 partitions.

List of disks and partitions

In the same line, creating a new filesystem on top of a particular block device (e.g. a partition) was done by "editing" such device and just selecting "Format Device" in this screen, that would eventually replace the existing filesystem in the device (if any) with a new one.

Creating a filesystem by editing a blk device

With such approach, there is currently no way to represent a filesystem that spreads over several block devices. Moreover, the filesystem itself is never represented as an entity or device on its own behalf.

Even worse, that screen (and in general all workflows that imply filesystems, like creating a partition) mixes concepts of the filesystem itself with others that are actually related to the concrete underlying block device, like encryption or the partition id.

Moreover, that screen can hardly be reused as-is for editing a multi-device filesystem if we take into account that some combinations of selections (like marking "do not format" for a new device) means exactly the opposite, that is, not having a filesystem at all.

Other multi-device technologies

As pointed at the start of the document, Btrfs tries to blend into the filesystem itself some of the features of LVM and software RAIDs. Those two technologies allow to combine several block devices and are already supported in the Partitioner.

The following screenshot shows the current list of software RAIDs present on the system. The RAIDs are a block device on themselves, so they have their own device name (like /dev/md0) and can be represented in a similar way to disks or partitions.

List software RAIDs

To see the list of block devices that are combined into each RAID, the "Used Devices" tab of the corresponding RAID must be used.

Devices in a software RAIDs

It is worth pointing that the list of disks does not offer any clear visual indication about which disks or partitions are used by each LVM or RAID (/dev/md1 is not mentioned in the rows representing sda and sdb).

Disks list with no info about RAIDs

The pointless Btrfs tree entry

Since LVM and RAID and even bcache have their own entries in the Partitioner tree, it seems to make sense to follow the same approach in order to represent the multi-device nature of Btrfs filesystems.

Suprisingly enough, there is already such "Btrfs" entry in the tree... but it looks like a half-baked list of all the block devices formatted as Btrfs in the system.

List of Btrfs devices

Such screen contains no single reference about the multi-device nature of Btrfs. Even worse, it offers close to zero functionality, no buttons to add, delete or modify the filesystems beyond an "Edit" button that can only be used to edit the list of Btrfs subvolumes.

List of Btrfs subvolumes

Allocating that button was probably the only reason to have a separate "Btrfs" entry.

Subvolumes as a (rather hidden) plain list of names

And managing the Btrfs subvolumes is, as a matter of fact, other of the areas in which the current approach of the Partitioner fails.

The list of subvolumes for a given filesystem is accessible in two places - using the mentioned button in the Btrfs section of the Partitioner and also while editing a Btrfs filesystem via the "Subvolume Handling" button (see screenshot in the section "Filesystems coupled to their block devices").

Editing subvolumes

In such pop-up driven interface the "@" subvolume is not represented at all, since there is nothing like nesting in that UI (apart from the nesting that can be inferred by the subvolume paths). Instead of clearly representing such special subvolume, a "@" character is automatically added to each new subvolume, pretending it's just a prefix. The way that "prefix" is managed turns to be quite confusing (and usually bogus when editing a filesystem that does not contain such special subvolume).

There is no visual indication of which subvolume is the default one (not to be confused with the toplevel subvolume) or other relatively important properties of the subvolumes like their ids, their mount options, etc. Only the "noCow" property is somehow hammered into the interface.

The snapshoting feature of Btrfs is heavily based on subvolumes. But that connection is not visible at all in the Partitioner. If during installation the "Enable Snapshots" is checked, a .snapshots subvolume will be created in the root filesystem. But that subvolume is not visible in the corresponding list.

Last but not least, the management of shadowed subvolumes is not exactly intuituve, with many things happening behind user's back. If a subvolume becomes shadowed, it will simply disappear from the list with no trace and will re-appear as soon as the device that was shadowing it is modified or deleted.

Changes implemented in the UI to accommodate the features

This section describes how the Partitioner UI will be adapted to offer support for multi-device Btrfs filesystems and to improve the management of Btrfs subvolumes. There is no need to implement both things at once. Quite the opposite, multi-device filesystems is a clear priority that should be addressed as soon as possible while the management of subvolumes is not that relevant at this point of time.

This section does not list all the options that were considered, only the ones that are planned to be actually implemented. See the next section to check some discarded changes.

Changes in the general list of devices

To better reflect the interactions between the different devices and to make possible to visualize the multi-device filesystems, some changes would be needed in the general lists of devices that are currently used all along the Partitioner.

Let's take an example with several formatted partitions, a bcache0 device based on sda1 and sda2, an MD RAID formed by sdb3 and sdb7 and an LVM based on the sdc disk. That's all visualized in the following devices graph.

Example devicegraph

The classical list of the partitioner will display all those devices like the following picture.

Current full list of devices

With so many different kinds of possible rows (disks, partitions, RAIDs, LVM... and now bcache and multi-device filesystems), it's necessary to unify the "Type" and "FS Type" columns into a new one which makes a better job informing about the relationships of the different devices.

New full list of devices

With that change, multi-device filesystems could be represented as an extra row in the table of devices, with its corresponding block devices being labeled as "Part of...". The information about filesystem label and mount point will only be presented in the row corresponding to the multi-device filesystem, not in each of its individual block devices. See the following example in which the root filesystem spreads over sdb2 and sdb7.

List including a multi-device Btrfs

The logic of the new column for each possible element is described in this gist.

Revamping the Btrfs tree entry

The current Btrfs tree entry is a lost opportunity to manage the Btrfs filesystems as entities on their own. That section of the Partitioner could be revamped to contain a table with the same behavior exposed for other types of devices like RAIDs. With a button to add a new Btrfs filesystem, a button to edit existing Btrfs filesystems (a real edit, not the pop-up for subvolumes), etc.

On this revamped section, each Btrfs filesystem would have an overview page describing its details and a "used devices" tab similar to the one used for RAID devices.

Last but not least, the tree entry could contain subentries for every Btrfs filesystem, just like "Hard Disks" contains a subentry for every disk or "RAID" contains one for every RAID device.

All that should raise the Btrfs filesystem to the category of first-class citizens in the Partitioner UI.

In the general list of devices described in the previous subsection only the multi-device filesystems would be represented in their own extra row. That's not the case in the Btrfs section of the partitioner. All the Btrfs filesystems, would be listed in this section, no matter whether they are multi-device or not. That would allow to add more block devices to a Btrfs filesystem that was initially defined only on top of one block device.

Do not allow to edit the individual block devices

There is no button in the Partitioner to delete a traditional filesystem. Instead, a filesytem will be implicitly deleted if the corresponding block device is encrypted, re-formatted or used for any other purpose like adding it to a RAID or LVM. Following the same approach for multi-device filesystem would lead to problems and inconsistencies in the user experience. Take the following situation as example:

  • Starting point: a Btrfs over sda1 and sdb1.
  • The user edits sda1 and marks it to be formatted as Ext4. That implicitly deletes the Btrfs.
  • Then the user uses sdb1 for something else, like adding it to a LVM setup.
  • Afterwards, the user edits sda1 again and chooses "Do not Format".

In the regular case, "Do not Format" means keeping the filesystem that is currently in sda1 in the real disk. But that cannot be done because the other device in the filesystem (sdb1) is not longer available. So the exact meaning of "Do not Format" would become unpredictable. The level of confusion can only grow with every subsequent user action.

To avoid that kind of pitfalls, the multi-device Btrfs must work in the same way than RAID or LVM regarding deletion and regarding usage of its individual devices. The UI must contain an explicit button to delete multi-device filesystems and editing the individual block devices must be prevented with a warning similar to this.

Prevent editing Btrfs block devices

Of course, that warning only applies to Btrfs filesystems that spread over several block devices.

Edit screen for Btrfs filesystems

Since the standard dialogs for editing block devices mix logic about modifying the device itself and defining its filesystem, those dialogs cannot be used when the user clicks "edit" for one of the entries in the revamped Btrfs section.

A new simplified version of that main dialog would be needed. It would likely look similar to the following mockup.

Dialog to edit a Btrfs filesystem

Better management of subvolumes

TO BE WRITTEN: we have still not discussed how to improve the UI in order to manage subvolumes in a more convenient way.

Discarded or postponed changes in the UI

This section lists several changes in the UI that could have been done to accommodate the new Btrfs features but that were finally discarded in the short term. They are documented for completeness and because some of them could be reconsidered in the future.

The "File Systems" tree entry

Going one step further, the current "Btrfs" entry in the left tree could be replaced with a "File Systems" one that will contain a list with all the filesystems, including the multi-device Btrfs ones but also any other traditional filesystem of any type.

This will be the only view in which the traditional filesystems (backed by only one block device) would be visible as an entity on their own, instead of just some kind of property of the underlying block device.

This option was discarded because it would add too much duplication to the left tree and to the partitioner in general with no clear gain. The only case in which this extra section would be useful to do something that cannot be currently done would be with Btrfs filesystems.

More information about the device while editing it

The initial plan was to not have an explicit button to delete a Btrfs filesystem, since such button is not offered for any other kind of filesystem. According to that plan, it would be possible to just "edit" any of the block devices that are part of multi-device Btrfs or to simply use that block device for any other purpose (like adding it to a LVM setup or to a RAID). Any of those changes would imply the deletion of the previous Btrfs filesystem.

Imagine for example a Btrfs filesystem on top of sda1 and sdb1. Editing sda1 and choosing to format it with a XFS filesystem will destroy the whole Btrfs filesystem, which also affects sdb1.

But with the traditional UI, the consequences of selecting each possible option while editing an existing block device are not always clear. The existence of multi-device filesystems only makes this worse. Thus, the initial plan was to enhance the edit dialog for a block device with some information about the current state of the device and adapting the labels of the different options to better explain their implications.

For example, imagine the process of editing a partition that already contains a Btrfs filesystem. The edit screen could look like the following screenshot. Apart from the extra explanation about the status (at the top), note how the traditional option "Do not Format" has been re-labeled as "Keep Current Filesystem".

Improved dialog to edit block device

This is a summary of all the changes that would be needed, depending on the real current content of the block device in the disks.

  • New block device or device that is neither encrypted or formatted.
    • No changes.
  • Formatted device (encrypted or not).
    • Add a sentence like "Currently [Encrypted and] Formatted as XFS".
    • For multi-device it would be something like "Currently [Encrypted and] Part of a Btrfs".
    • "Do not Format" changed into something like "Keep Current Filesystem".
    • No changes to the "Encrypt Device" label, since it's only enabled when "Format Device" is chosen.
  • Encrypted but not formatted device.
    • Add sentence like "Currently Encrypted".
    • No changes in labels for formatting (i.e. use "Do not Format").
    • If "Format Device" is chosen, the label for the encrypt checkbox should still be "Encrypt Device". But if "Do not Format" is chosen, then it must change to something like "Keep Current Encryption" (see below).
  • Any other case
    • Irrelevant, it's impossible to edit block devices in any other case.

The labels for the encrypt checkbox in the case of encrypted but not formatted look strange... because the behavior is actually that inconsistent. When choosing to format and encrypt, a new encryption layer is created replacing the old encryption, but "Do not Format" keeps the original encryption layer. So, in the traditional UI, the encryption checkbox indeed means different things depending of the selection in the format radio button.

Rethinking the dialogs to create/edit filesystems

The solutions described in the section "Changes implemented in the UI to accommodate the features" fix the main problems in the short term but they expose some inconsistencies. Imagine a partition formatted as Btrfs, clicking "edit" for the partition and for its corresponding Btrfs filesystem would result in two different dialogs.

The experience could be smoother and make more sense (at several levels) with a better organization of the dialogs and workflows to format a device (or a set of devices, now that multi-device filesystems are possible) and to modify existing filesystems.

Let's take a look at what could be a new workflow for creating and formating a new partition. It implies the same number of steps than in the classic workflow. But none of new steps mixes logic referred to the partition with logic related to the filesystem itself. That means filesystem-specific dialog(s) can be directly reused to create and edit multi-device filesystems.

The first two screens remain identical. One for defining the size of the partition.

Partition creation step 1

And the role selection which allows to predefine some of the options for the following screen(s).

Partition creation step 2

But the third screen changes radically and groups all the options that affect the partition itself and has nothing to do with the configuration of the filesystem to be created. If "Do not format" is choosen, this would be the last screen. Take into account this is a mockup and the final distribution and labels may change.

Partition creation step 3

The last screen if a filesystem is going to be created will only contain options that are indeed applicable to the filesystem in any case, no matter if the filesystem sits on a disk, on a partition or it's a multi-device filesystem. Once again, this is only a preliminary mockup.

Partition creation step 4

Creating a new multi-device filesystem would consist in two steps. A first one to select the block devices (pretty similar to the first screen displayed when creating a RAID) and the mockup above this paragraph as the second and last screen.

This new approach with separate steps for the settings of the block device (like encryption or partition ID) and the configuration of the filesystem (formatting and mounting options) may have a drawback when editing an existing block device. If the device is not going to be encrypted, that "edit" operation has been traditionally handled in a single screen (as already seen in the section "Filesystems coupled to their block devices"). Now, the two final steps of the workflow described for the creation will very often be necessary for editing, except in those cases in which the block device does not contain a filesystem and it's not going to be formatted.

On the bright side, with the information being provided in two more fine-grained steps, the dialogs are more clear and the improvements described in the section "More information about the device while editing it" would make even more sense.

For example, imagine the process of editing a partition that already contains a Btrfs filesystem. The first screen could already show some information and more adapted labels. For example, the option "Do not Format" could be re-labeled as "Keep Current Filesystem" if the partition is not going to be encrypted. Note also the extra warning in the "Encrypt Device" checkbox.

Partition edit step 1

And the formatting options could be disabled if the current filesystem is going to be kept.

Partition edit step 2 (option A)

Even more, such options could simply not be present at all. Once again, these are just mockups and the finally layout should be enhanced.

Partition edit step 2 (option B)

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