[RFC PATCH 3/3] Documentation: Add NSD filesystem documentation

[Ayhan Aydin <nsd.project.dev@gmail.com>]

Add initial documentation for the Neural Storage Driver
covering architecture, sysfs interface, and performance.

Signed-off-by: Ayhan Aydin <nsd.project.dev@gmail.com>
---
 Documentation/filesystems/nsd.rst | 77 +++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)
 create mode 100644 Documentation/filesystems/nsd.rst

diff --git a/Documentation/filesystems/nsd.rst b/Documentation/filesystems/nsd.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+========================================
+Neural Storage Driver (NSD) - v1.0.0
+========================================
+
+Overview
+========
+NSD is a learning prefetcher for the Linux kernel page cache. It monitors
+I/O patterns via a hook in :c:func:`filemap_read` and prefetches pages
+ahead of the application using :c:func:`page_cache_sync_readahead`.
+
+Unlike the kernel existing readahead (fixed window), NSD builds
+a synaptic Markov chain model of access patterns at 4 KB region
+granularity. It detects sequential strides, repeating patterns, and
+learned transitions.
+
+Architecture
+============::
+
+    Application
+         |
+         v
+    filemap_read()  <-- NSD hook
+         |
+         +---> nsd_notify_read()
+                   |
+                   +---> Synaptic Table (Markov chain)
+                   |        |
+                   |        +---> Region hash + stride predictor
+                   |
+                   +---> Prefetch Worker
+                            |
+                            +---> page_cache_sync_readahead(WILLNEED)
+
+Sysfs Interface
+===============
+/sys/kernel/nsd/::
+
+  stats          Current statistics (hit rate, prefetch count, etc.)
+  observe_only   0=active, 1=observe-only (no prefetch)
+  penalty        Enable penalty weakening on waste
+  waste_track    Enable waste tracking
+
+Statistics fields::
+
+  prefetched    Total pages prefetched
+  used          Prefetched pages that were actually accessed
+  wasted        Prefetched pages never accessed
+  hit_rate_real Used / Prefetched ratio
+  stride_preds  Stride predictions made
+  chain_depth   Average chain prediction depth
+  synapse_ents  Current synapse table entries
+  ring_events   Pending ring buffer events
+
+Module Parameters
+=================
+  observe_only  Start in observe-only mode (default: false)
+  penalty       Enable penalty on waste (default: true)
+  waste_track   Enable waste tracking (default: false)
+
+Performance
+===========
+Tested on: x86_64, SATA SSD, kernel 7.0.0::
+
+  Workload                     Improvement
+  ---------------------------  -----------
+  SQLite FTS (4 GB table)      -18.8% query time
+  Sequential 64K buffered      +22.6% throughput
+  Random 4K buffered           +1.1% (noise)
+
+The prefetcher achieves 98% real hit rate on SSD workloads.
+
+See Also
+========
+Documentation/filesystems/fscache.rst
+Documentation/filesystems/squashfs.rst
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