Add kv state store

.gitignore

@@ -2,6 +2,7 @@
 !scripts/*.py
 .DS_Store
 /target
+/wal_kv
 /wal_files
 digest.txt
 *.csv

README.md

@@ -69,7 +69,7 @@ when you pass `true`.
 ### Advanced Configuration
 
 ```rust
-use walrus_rust::{Walrus, ReadConsistency, FsyncSchedule, enable_fd_backend};
+use walrus_rust::{Walrus, ReadConsistency, FsyncSchedule};
 
 // Configure with custom consistency and fsync behavior
 let wal = Walrus::with_consistency_and_schedule(
@@ -85,7 +85,7 @@ wal.append_for_topic("events", b"event data")?;
 
 - **Read consistency**: `StrictlyAtOnce` persists every checkpoint; `AtLeastOnce { persist_every }` favours throughput and tolerates replays.
 - **Fsync schedule**: choose `SyncEach`, `Milliseconds(n)`, or `NoFsync` when constructing `Walrus` to balance durability vs latency.
-- **Storage backend**: FD backend (default) uses pread/pwrite syscalls and enables io_uring for batch operations on Linux; `disable_fd_backend()` switches to the mmap backend.
+- **Storage backend**: FD backend (default) uses pread/pwrite syscalls and enables io_uring for batch operations on Linux.
 - **Namespacing & data dir**: set `WALRUS_INSTANCE_KEY` or use the `_for_key` constructors to isolate workloads; `WALRUS_DATA_DIR` relocates the entire tree.
 - **Noise control**: `WALRUS_QUIET=1` mutes debug logging from internal helpers.
 

benchmarks/batch_scaling_benchmark.rs

@@ -148,19 +148,16 @@ fn configure_storage_backend() {
     {
         match selection.as_deref() {
             Some("mmap") => {
-                walrus_rust::disable_fd_backend();
                 println!("Storage backend: mmap");
             }
             Some("fd") | Some("io_uring") | Some("uring") | Some("file") | None => {
-                walrus_rust::enable_fd_backend();
                 println!("Storage backend: fd");
             }
             Some(other) => {
                 println!(
                     "Unknown storage backend '{}'; defaulting to fd (io_uring) backend.",
                     other
                 );
-                walrus_rust::enable_fd_backend();
             }
         }
     }

benchmarks/multithreaded_benchmark_batch.rs

@@ -233,19 +233,16 @@ fn configure_storage_backend() {
     {
         match selection.as_deref() {
             Some("mmap") => {
-                walrus_rust::disable_fd_backend();
                 println!("Storage backend: mmap");
             }
             Some("fd") | Some("io_uring") | Some("uring") | Some("file") | None => {
-                walrus_rust::enable_fd_backend();
                 println!("Storage backend: fd");
             }
             Some(other) => {
                 println!(
                     "Unknown storage backend '{}'; defaulting to fd (io_uring) backend.",
                     other
                 );
-                walrus_rust::enable_fd_backend();
             }
         }
     }

docs/architecture.md

@@ -137,12 +137,9 @@ sequenceDiagram
 
 ## Backend Selection
 
-- `enable_fd_backend()` flips a process-wide atomic that forces new blocks to
-  use the fd-backed storage (and therefore enables `io_uring` batching on Linux).
-- `disable_fd_backend()` reverts to mmap-backed accesses; batch writes fall back
-  to sequential writes and batch reads use direct mmap parsing.
-- The default is FD+`io_uring` when supported; non-Linux builds automatically
-  live on the mmap backend.
+- The FD backend uses fd-backed storage and enables `io_uring` batching on Linux.
+- The mmap backend uses mmap-backed accesses; batch writes fall back to sequential writes and batch reads use direct mmap parsing.
+- The default is FD+`io_uring` when supported; non-Linux builds automatically live on the mmap backend.
 
 ## Concurrency & Synchronization
 

docs/batch_writer.md

@@ -201,7 +201,7 @@ pub fn write(&self, data: &[u8]) -> std::io::Result<()> {
 
 ### Required
 - `io-uring` crate (already in use)
-- FD backend must be enabled (`enable_fd_backend()`)
+- FD backend (enabled by default on Linux)
 - Linux kernel with io_uring support
 
 ### Not Required