= Buffer cache access trace
Let $A,B,C$ be three distinct <buffer cache keys> on one device. Use two initially empty slots, <LRU replacement> refreshed at each completed request, and synchronous successful <buffer cache writeback> of a dirty victim. Each request acquires its target, finishes any fill, reads or partially modifies it, and releases the client reference. An explicit pin on $A$ remains after request 2 through request 7; pin and unpin do not refresh recency. No other references, invalidations, background writes or durability operations occur. Initial backing contents are represented by the <integers> $A=10,B=20,C=30$; partial writes change the tracked value of $A$ to 11 and then 12 while preserving the unshown remainder.
\Image[/buffer-cache-pinned-dirty-eviction-trace.png]
{title=Exact <cache hits>, <cache misses>, retained references and <buffer cache evictions> in a two-slot <buffer cache>}
The eight data requests are read $A$, write $A=11$, read $B$, read $C$, read $A$, write $A=12$, read $C$, and read $B$ after unpinning $A$. They yield four <cache hits>, four <cache misses>, four device reads and one device write. The pin and unpin are lifetime operations, excluded from the data-request count. At the read of $C$, $A$ is older than $B$ but pinned, so the <clean buffer> for $B$ is evicted instead. Reading $A$ then returns 11 without device access even though its backing value is still 10. The second write supersedes 11; reading $C$ makes $A$ the older entry again. After unpinning, the final read of $B$ writes back 12 before evicting $A$. After that request, backing $A$ contains 12, while the resident keys are $C,B$.
This example exposes the value of retaining modified contents: a <cache hit> can return a newer value than a device read would. It also exposes the cost of <reference counting> constraints: a pinned older entry forces a different victim. An independent capacity sweep gives two <cache hits>, six <cache misses> and two device writes with one slot and no pin, but three <cache hits>, five <cache misses> and two device writes with two slots and no pin. With one slot and the stipulated pin, the read of $B$ cannot proceed. With three slots, both pin choices give five <cache hits>, three <cache misses> and no eviction-triggered write; $A$ remains dirty with value 12, so the absence of a device write does not establish <file-system transaction durability>.
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