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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.

Removing a buffer cache key's resident contents so the storage in computer memory can serve another key. An unreferenced clean buffer can be reused directly in the buffer cache state model; an unreferenced dirty buffer requires successful buffer cache writeback first. A pinned buffer cannot be chosen regardless of cleanliness. LRU replacement ranks eligible candidates; it does not override lifetime protection. If every candidate is retained, a cache miss cannot obtain space until something releases a reference or capacity changes. This is a resource shortage, not permission to discard live contents.

A pinned memory object is a normally movable memory object whose address must remain unchanged during a specified operation. Pinning permits a pointer to remain usable when the code consuming it cannot participate in the runtime's relocation of references. The pinning lifetime is part of that guarantee: an address retained beyond the protected operation need not remain valid. Microsoft's language reference specifies that a fixed statement prevents relocation during its body and restricts the declared pointer to that statement.^[ref]

For an original layout example, consider96 bytes with a pinned16-byte object at offsets $[32,48)$ and one other16-byte live object. The remaining64 bytes are free. Even if the other object can move anywhere, the pinned object divides the address range into regions of32 and48 bytes. A56-byte contiguous allocation cannot fit. Moving the other object into $[0,16)$ gives free intervals $[16,32)$ and $[48,96)$, whose largest size is48 bytes. If both live objects could move, placing them in $[0,32)$ would instead leave a64-byte interval. Pinning can therefore limit memory compaction despite ample total free capacity.

The numerical layout omits allocation headers and memory alignment; a real allocator must include them. Pinning an object's address also does not by itself specify how its virtual memory pages are backed. The garbage collector's object-relocation guarantee and an operating system's physical-page residency rules concern different layers. Likewise, a pinned buffer in a storage cache may mean protection from eviction rather than this object-relocation guarantee.

Once the protected operation is finished, releasing its pin can restore the runtime's freedom to move the object during a later collection. The program still has to obey the language's reference-lifetime rules: releasing a pin does not convert an old unmanaged address into a relocatable reference. The general garbage collection process must update references when it moves surviving objects.^[ref]