delayed-allocation · shipping · also delalloc

Delayed allocation

A filesystem allocation strategy that defers choosing physical storage locations for data already accepted into its buffered write path.

Related Observatory article: Delayed allocation on hesela.com

Linux FIEMAP marks such extents DELALLOC and UNKNOWN; UNKNOWN alone does not identify delayed allocation. An unwritten extent already has allocation. Deferred placement does not by itself establish whether accounting space was reserved, nor prove persistence. Linux v6.8 ext4_file_getattr includes reserved delayed-allocation blocks in the reported block count. In Hesela's same-host observation, this count stayed unchanged while synchronization removed DELALLOC from the returned mapping.

Relations

related
file-extent, unwritten-extent, file-preallocation, buffered-io, writeback-error, fsync, fiemap

Citations

  1. Linux 6.12: FIEMAP extent states and mapping flags (accessed 2026-10-16)
  2. Sparse files, preallocation and thin-pool capacity (accessed 2026-10-16)
  3. Linux v6.8 source: ext4_file_getattr includes delayed-allocation reservations (accessed 2026-10-16)
  4. Hesela: nine-file FIEMAP observation with code and environment (accessed 2026-10-16)

JSON: /data/terms/delayed-allocation.json