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Joined 2026-09-21 Message
Buffer cache 2026-10-13
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]
File System Implementation. pp. 14–16: caching and buffering
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.
Hard disk drive 2026-10-31
A data storage device that records information magnetically on rotating platters. Accessing a sector can require moving a head to its track, waiting for the sector to rotate beneath it, and transferring the data. These contributions are seek time, rotational latency and data transfer time.[ref]
Operating Systems: Three Easy Pieces by Remzi H. Arpaci-Dusseau and others. Chapter 37, Hard Disk Drives, §37.3, seek, rotation and transfer phases
Online text: pages.cs.wisc.edu/~remzi/OSTEP/vm-beyondphys.pdf
Operating Systems: Three Easy Pieces by Remzi H. Arpaci-Dusseau and others Hard Disk Drives 2026-10-31
Online chapter: pages.cs.wisc.edu/~remzi/OSTEP/file-disks.pdf
Chapter 37 of Operating Systems: Three Easy Pieces by Remzi H. Arpaci-Dusseau and others. Section 37.3 separates seek, rotational delay and transfer time; the later scheduling discussion compares shortest-seek and directional-sweep policies. The parent record supplies the authors and version metadata.
Operating Systems: Three Easy Pieces by Remzi H. Arpaci-Dusseau and others Mechanism: Address Translation Created 2026-10-14 Updated 2026-10-31
Online text: pages.cs.wisc.edu/~remzi/OSTEP/vm-mechanism.pdf
Online text: pages.cs.wisc.edu/~remzi/OSTEP/vm-tlbs.pdf
Page replacement 2026-10-14
Choosing a resident memory page to evict when another memory page needs an occupied page frame. An unused page frame needs no eviction. A dirty memory page requires preservation before reuse, accounted for separately as page writeback. A page replacement policy balances likely future reuse against the overhead of tracking past memory references.
The opportunity comes from temporal locality: retaining a recently accessed memory page can spare a later nonresident page fault. Spatial locality helps consecutive accesses reuse the same memory page until they cross its boundary. A working set describes recent demand, while the resident set describes what is actually present. A small active footprint can fit in the available page frames even when the whole address space cannot. These are useful workload tendencies, not guarantees about future memory references.[ref]
The page-reference trace below uses the counterexample discussed in Operating Systems: Three Easy Pieces by Remzi H. Arpaci-Dusseau and others (chapter 22, p. 5) and makes the decisions explicit. FIFO page replacement ranks arrival time, LRU replacement ranks last access, CLOCK page replacement samples recent use, and optimal page replacement knows the remaining sequence in advance. Count misses first; page-replacement cost accounting separately counts page writeback and explains why fewer misses need not mean less total service time.
// Destination: computer-science.bigb