foyer disk-engine write-path tuning. Defaults reproduce foyer’s own
BlockEngineConfig defaults (flushers=1, buffer_pool_size=16 MiB),
with the submit-queue threshold pinned to 2x the buffer pool – which
foyer’s doc comment describes as its intended default but which 0.22 no
longer applies automatically (its actual default is 1x, see
BlockEngineConfig::new) – so existing callers/tests are unaffected
unless they opt into a different tuning.
Marker filename holding the decimal DISK_FORMAT_VERSION, stored alongside
foyer’s own foyer-storage-direct-fs-* region files inside --disk-path.
The name does not collide with foyer’s prefix, so foyer’s recovery ignores it.
On-disk format version for the foyer disk tier. The serialized value layout
(CachedBlock’s Code impl) carries no self-describing version, so a layout
change would otherwise misdecode entries recovered from a persisted store on
restart (see #1287, #1283). On startup the store directory is wiped iff the
persisted marker does not match this constant.