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A memory cache of device blocks indexed by device identity and block number. A hit avoids slow device access; a miss obtains a buffer and reads the block. Dirty buffers carry modified data that must be written before reuse, and pinned buffers cannot be evicted while an operation uses them. Deferred writes improve throughput but need explicit durability handling.

A buffer cache can hold both file data blocks and storage containing file metadata, such as inodes and block allocation bitmaps. Reusing a buffered device block saves repeated input-output; retaining several modifications before buffer cache writeback can combine them into one transfer. Section 40.7 of Operating Systems: Three Easy Pieces by Remzi H. Arpaci-Dusseau and others explains these benefits for file systems.^[ref]

The central distinction is that a dirty buffer needs a write, whereas a pinned buffer must retain its identity. A clean buffer can still be pinned, and a dirty buffer can have no client references. Neither releasing a reference nor obtaining a cache hit establishes file-system transaction durability.

Use one fixed-size buffered device block per buffer cache key, serialized operations, successful reads and writes, and no outside changes to the backing contents. Record an absent state or a reserved key with validity $v$, dirty flag $D$, and retained-reference count $r$. The bits $v,D$ describe contents, while the nonnegative integer $r$ describes lifetime. Every dirty buffer is valid, so $D=1$ implies $v=1$. Reservation begins at $(v,D,r)=(0,0,1)$; successful fill changes only $v$ to 1. Modifying a valid retained structure sets $D=1$. Releasing a caller changes $r$, not $v$ or $D$.

The first diagram separates contents transitions from reference transitions. Buffer cache writeback temporarily retains the structure while transferring its contents; that internal reference is acquired and released inside the single serialized operation shown. Buffer cache eviction requires a valid clean buffer with $r=0$. The diagram omits read errors, explicit cancellation of invalid reservations, concurrent writes, invalidation and journal ordering. In particular, a journal can forbid otherwise possible buffer cache writeback until its write-ahead logging dependencies are satisfied.

For a small independent check, bound $r$ to $0,1,2$. Starting absent, allow reservation, acquisition, successful fill, modification, successful buffer cache writeback, release of a valid retained structure, and eligible buffer cache eviction. Exactly nine abstract states are reachable: absent, two invalid reservations with $r=1,2$, and six valid states combining $D=0,1$ with $r=0,1,2$. Every transition preserves $D\le v$; the only eviction source has $(v,D,r)=(1,0,0)$. This bounded finite-state machine check supports these safety rules, not a claim that work always completes.