# `src/io/mod.rs` — background

## `WritebackFile`

Drop-in wrapper around `std::fs::File` for any call site that performs
large sequential writes (sweep, patch, mux, etc.). It implements `Write`
and `Seek` so existing code paths can swap `File` for `WritebackFile`
with no body changes. Internally it drives a `WritebackPipeline` that,
on Linux, drains dirty pages continuously at 32 MB granularity to avoid
the kernel's accumulate-then-burst flush behaviour. macOS and Windows
use a no-op pipeline — their default cache policies have not been shown
to exhibit the same pathology for this access pattern.

## `FileSectorSource`

The read-side dual of `WritebackFile` — it implements
[`crate::sector::SectorSource`] for an ISO file using direct
`pread`-equivalent calls so the kernel's own readahead policy runs
(which interleaves naturally with the concurrent writeback). It pairs
that with periodic `posix_fadvise(DONTNEED)` drops on the consumed
window so an 85 GB streaming ISO read doesn't fill the page cache and
starve the concurrent MKV write.

## `Pipeline` + `Sink`

The generic producer/consumer primitive used by sweep, patch, and mux
to overlap reads with writes via a bounded channel + dedicated consumer
thread.

## `byte_prefetcher`

The read-ahead producer feeding the mux pipeline for `io::Read`-backed
sources: a worker thread fills a recycled pool of buffers and ships
them through a channel, exposing `BytePrefetcher` / `PrefetchShell`.