Add btrfs balance option to low disk space notification

[mmmikko]

Problem Description

When the System Rollback feature detects low unallocated btrfs space (below 15%), it displays a notification warning users about potential file corruption and paused snapshot maintenance. However, the notification only suggests:

• Removing files from /home
• Purging software packages
• Moving databases/VMs/logs
• Using Rollback Dashboard

These suggestions address used space, but the actual problem is often fragmented btrfs chunk allocation - where plenty of free space exists within allocated chunks, but little unallocated space remains for new chunks.

Real-World Example

$ df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/nvme0n1p6   86G   36G   46G  44% /

$ sudo btrfs filesystem usage /
Device size:           85.43GiB
Device unallocated:     6.41GiB  (~7.5%)
Free (estimated):      45.92GiB

The user sees 46GB free (56%), but kfocus correctly warns about only 7% unallocated. The fix is simply:

sudo btrfs balance start -dusage=50 /

This reclaims unused space from partially-filled chunks back to the unallocated pool.

Suggested Improvements

1. Add "Run Balance" button to the kdialog notification that executes the balance command (with pkexec for privileges)

2. Update notification text to explain the difference between "free space" and "unallocated space" and mention that balance can help

3. Consider proactive balancing in _maintainBtrfsFn() - automatically run a light balance (e.g., -dusage=30) when unallocated drops below 20%, before it becomes critical

Affected Files

• /usr/lib/kfocus/bin/kfocus-rollback-backend - _lowDiskWithHomeMsg and _lowDiskWithoutHomeMsg message templates
• /usr/bin/kfocus-focusrx-dbus-dispatch - kdialog launcher (would need new button handler)
• /usr/lib/kfocus/bin/kfocus-focusrx-system - _maintainBtrfsFn() for proactive balancing

Why This Matters

This is a common btrfs behavior that confuses users. The kfocus team already did the right thing by monitoring unallocated space (most distros don't), but the current remediation suggestions don't address the root cause. Users delete files unnecessarily when a simple balance would fix the issue.

[ArrayBolt3]

Thanks for the report! The button you're suggesting adding is actually already there, though it's not in the popup itself, and it's usually unnecessary for users to press.

System Rollback automatically check if the disk needs balancing once every four hours. If it is determined that the disk does need balancing (triggered by at least 5GB of additional unallocated space being consumed or 5GB of available (not unallocated) space freed since the last check), a balance is done that reclaims blocks that are 70% full or lower. This prevents constant balancing that could result in wear-and-tear on the SSDs, while also ensuring that the disk is balanced if enough churn has occurred to warrant it. As a result, if the user is seeing this message, a balance is most likely not needed since the system has probably already done it for them. This is why we recommend the removal of files and snapshots instead.

However, a balance button is provided. In the System Rollback Dashboard, under "Global Actions", there is a "Free Up Disk Space" button. Clicking that will offer "Quick Clean" and "Deep Clean" options. The "Quick Clean" option will make System Rollback run through the BTRFS maintenance routine immediately, rather than waiting for the next automatic check time. This will skip the balance if the disk's contents have not changed enough, but in such an instance a balance would probably not do much (if anything) anyway. (We don't explicitly mention the term "balance" in the UI, because it's a technical term specific to BTRFS, and we wanted the UI to be accessible to anyone, including people who aren't familiar with the intricacies of BTRFS.)

The current UI for attempting a balance when the notification popup appears looks like this:

[mmikowski]

@ArrayBolt3 I think this is an excellent answer, but I think @mmmikko illustrates some important points:

1. We should be probably suggesting Quick Clean as the first course of action in the pop-up dialog.
2. It should probably be a little clearer what "Quick Clean", perhaps by being described as "balancing the disk" in the Free Up Disk Space dialog.

What say you?

[ArrayBolt3]

1. We should be probably suggesting Quick Clean as the first course of action in the pop-up dialog.

It might work in some instances. In instances where the disk hasn't had its space consumption change by 5 GiB though, it won't do much of anything. It probably can't help to suggest starting with that though.

2. It should probably be a little clearer what "Quick Clean", perhaps by being described as "balancing the disk" in the Free Up Disk Space dialog.

Sure, we could mention that, as long as it's not confusing to users who don't know what "balancing" means.