Pixels uses an immutable columnar file + Visibility chain MVCC architecture: writes are append-only, and deletes only set marks in the in-memory TileVisibility (Deletion Chain + baseBitmap). The existing Memory GC reclaims expired Deletion Chain blocks in memory, but the physical files themselves never shrink. As deletes accumulate, "ghost rows" inflate storage and degrade scan performance. Storage GC addresses this by rewriting physical files with high deletion ratios, reclaiming storage space while guaranteeing that in-flight queries are not affected.

• gcSnapshotBitmap Production: Extend Memory GC to produce precise deletion bitmaps at safeGcTs.
• Checkpoint with Precomputed Bitmaps: Serialize gcSnapshotBitmaps directly to checkpoint file.
• Scan & Group (S1) — invalidRatio Calculation: Identify candidate files and group them for rewrite.
• Data Rewrite (S2): Read old files, filter out ghost rows, write new compact file.
• Dual-Write (S3): Keep new/old Visibility in sync during the transition window.
• Visibility Sync (S4): Migrate Deletion Chain items (ts > safeGcTs) from old files to new file.
• Index Sync (S5): Update MainIndex and SinglePointIndex to point to new file locations.
• Atomic Switch & Deferred Cleanup (S6): Commit the rewrite and safely retire old files.