[DenisCDev]

big-btrfs-balance runs btrfs balance start -dusage=50 -musage=50 five minutes after every boot. After the first run, the leftover data sits in one partly filled chunk. That chunk matches -dusage=50 again on the next boot, so the balance moves it into a fresh chunk, which is again partly filled. Nothing gets freed and the same work repeats on every boot, at the moment people are opening a game or a project. It also competes at normal I/O priority on SSDs and NVMe drives: 60-ioschedulers.rules picks kyber there, and kyber has no I/O priority handling (block/kyber-iosched.c in v6.17 has no ioprio reference), so ionice -c 3 changes nothing.

A balance can only free a chunk when the allocated-but-unused space of that type is at least one chunk. btrfs filesystem df -b reports it. This PR runs the data filter only when there is one data chunk of it, and the metadata filter only when there is one metadata chunk of it. A data chunk is 1 GiB; the metadata chunk size comes from /sys/fs/btrfs/<uuid>/allocation/metadata/chunk_size (256 MiB, the smallest metadata chunk, if that file is missing). The data chunk_size there is only the upper limit (10 GiB), so it is not used. On filesystems under 10 GiB the kernel makes chunks 10% of the size, rounded up to 16 MiB (fs/btrfs/volumes.c), so both thresholds are capped at that, using the size from btrfs filesystem usage -b; if that command fails they stay at 1 GiB and the metadata chunk size. The filters are still usage=50. Mixed data+metadata block groups keep the previous behavior.

Measurement

12 GiB btrfs loopback image, compress-force=zstd,noatime, --direct-io=on, Ubuntu 24.04 runner. Large and small files were written interleaved, then all small files and a third of the large ones were deleted. The script was run as-is, with / pointed at the test filesystem, three boots in a row, three repetitions each:

boot	original: written / returned to unallocated	patched: written / returned
1	1437-1438 MiB / 4048 MiB	1430 MiB / 4096 MiB
2	494-495 MiB / 0	0 / 0
3	495 MiB / 0	0 / 0

A second run used a 5 GiB image, where data chunks are 512 MiB, with 768 MiB of unused data space in two 25%-used chunks (run 40444622848):

boot	original: written / returned	patched: written / returned
1	148 MiB / 472 MiB	139 MiB / 520 MiB
2	8 MiB / 0 (2 chunks relocated)	0 / 0
3	9 MiB / 0 (2 chunks relocated)	0 / 0

On boot 2 of the 12 GiB run the patched script saw 541 MiB of unused data space and 255 MiB of unused metadata space, and started no balance. The original one moved one data chunk (-dusage=50 alone relocates 1 of 5 chunks and writes 487 MiB on each later boot).

Harness, layout generator and raw CSV/logs: https://github.com/DenisCDev/btrfs-balance-bench (run 40433262852 for 12 GiB, made with the first commit; on filesystems of 10 GiB or more the second commit leaves both thresholds unchanged). The decision was also checked with a stubbed btrfs against synthetic btrfs filesystem df -b and usage -b output for single, DUP, RAID1, mixed profiles, 256 MiB and 1 GiB metadata chunks, 2, 3, 5, 12 and 100 GiB filesystems, a missing sysfs file, a failing usage, empty output and the exact thresholds.

Limits

• This was not run on a BigLinux install. The script itself ran unmodified on a real btrfs filesystem on an Ubuntu runner, with the same kernel-facing commands, but not under the shipped big-btrfs-balance.service on a real BigLinux disk. Running bash -x /usr/bin/big-btrfs-balance on one would confirm the decision it logs.
• The test filesystem is small (a handful of chunks). On a large disk the first balance after many deletions costs more, and that cost is unchanged here. Only the repeated rewrite on later boots goes away.
• The threshold follows the current filesystem size. After growing a filesystem that started under 10 GiB, its older, smaller chunks can hold reclaimable space below the new 1 GiB threshold until more is freed.
• With RAID0/RAID10 data over several devices, chunks are larger than 1 GiB and some balances that cannot free anything still run, as before.
• On the test filesystem the metadata chunk is 256 MiB, the same as the fallback. The 1 GiB metadata chunk path (filesystems over 50 GiB) was only checked with the stubbed btrfs, not on a real disk.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Balance operations now run only when enough unused space is available in data or metadata chunks, using separate thresholds for each.
  • Data-and-metadata chunks are handled with both applicable filters.
  • On smaller filesystems, the thresholds are capped based on filesystem size.
  • Balance runs at low priority when a qualifying filter is selected.

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Walkthrough

The script selects Btrfs balance filters based on unused Data and Metadata space. It applies the balance function to the root filesystem and, when distinct, the home filesystem.

Changes: Btrfs balance selection

Layer / File(s)	Summary
Select filters and balance filesystems auto-rules-io/usr/bin/big-btrfs-balance	The script derives Data and Metadata thresholds, selects filters when unused space reaches the applicable threshold, and runs a low-priority balance only when at least one filter applies. Root and distinct home filesystem checks call the balance function.

Priority: Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: bigbruno

Merge Risk: Minimal · up to a754b

The script balances only when filters qualify. No merge-blocking issue was established.

Architecture risk: Low · up to a754b

The change affects 1 system.

Changed systems: auto-rules-io

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — auto-rules-io (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in auto-rules-io/usr/bin/big-btrfs-balance: Added balance, which derives chunk thresholds and unused-space totals, selects usage filters for eligible Data or Metadata space (or both for Data+Metadata), and runs the balance command only when at least one filter is selected. This replaces the prior unconditional use of both filters at the call sites.
• observed — Modified behavior in auto-rules-io/usr/bin/big-btrfs-balance: Root and distinct /home filesystems now call balance instead of directly running the command with both -dusage=50 and -musage=50.

Pre-merge checks: 4

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: skipping boot-time Btrfs balance operations when they cannot free a chunk.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
Review comments at @auto-rules-io/usr/bin/big-btrfs-balance:
- Line 21: Update the Data branch’s slack threshold so balance runs when small
data chunks can be consolidated, using a size-aware threshold or preserving the
previous behavior on smaller filesystems; keep the existing -dusage=50 filter
for eligible data chunks.

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