[GrootFS] GC threshold exceeds actual XFS store capacity — GC never fires

ai-assisted=yes

Current behavior

Thresholder runs at garden pre-start and computes clean.threshold_bytes from disk available space minus reserved. It never checks the actual XFS store filesystem capacity.

In the default configuration (modernCalculator), threshold_bytes equals store_size_bytes. However, XFS metadata overhead means the filesystem's actual reportable capacity is always slightly less than the backing file size. The GC trigger condition (committedQuota + totalVolumesSize >= threshold_bytes) becomes mathematically unsatisfiable — GC never fires.

Over time, orphaned layers from app push/delete cycles accumulate. The store eventually fills → ENOSPC → all container creation fails on that cell.

Root cause

// main.go — threshold computed from disk, never validated against store
config.Clean.ThresholdBytes = calc.CalculateGCThreshold()

The XFS store at config.StorePath is a fixed-size loop mount. Its actual filesystem capacity (as reported by statfs(2)) is always less than the computed threshold due to:

1. XFS metadata overhead (~11 MB): Even in the default config, threshold_bytes = store_size_bytes > actual XFS capacity. GC condition is unreachable on every cell.
2. Store/threshold mismatch (observed in production): In some cases the store is significantly smaller than the threshold (e.g. 35 GB store vs 268 GB threshold on a 284 GB disk). The exact cause of the undersized store is not determined, but the fix handles it regardless.

Impact

Every landscape running with grootfs.routine_gc=false (the default) is affected. Cells degrade over time as orphaned layers accumulate with no GC to reclaim space.

Desired behavior

Cap threshold_bytes at the store filesystem's total capacity (statfs(2) → Blocks * Bsize) so GC can always fire before the store is full.

Affected Version

garden-runc-release v1.86.0+ (pattern existed since thresholder introduction).

Fix

https://github.com/cloudfoundry/garden-runc-release/pull/394

Start in main.go at the threshold calculation and inspect how config.StorePath is used for the fixed-size XFS store. Review pull request 394 and the statfs(2) capacity calculation; done means the GC threshold cannot exceed the store filesystem capacity and GC can trigger before ENOSPC.