diff --git a/README.md b/README.md
index 8fc89de..022621d 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@ curl http://localhost:3000/?prefix=path/to/
 ### Operations
 
 ```bash
-# Rebuild index by scanning all volumes (disaster recovery)
+# Rebuild index by scanning all volumes (stop the server first)
 mkv -d /tmp/index.db -v http://vol1:8080,http://vol2:8080,http://vol3:8080 -r 2 rebuild
 
 # Rebalance after adding/removing volumes (preview with --dry-run)
diff --git a/src/db.rs b/src/db.rs
index f928d03..cd65587 100644
--- a/src/db.rs
+++ b/src/db.rs
@@ -76,13 +76,45 @@ impl Db {
 
     pub async fn list_keys(&self, prefix: &str) -> Result<Vec<String>, AppError> {
         let conn = self.conn.clone();
-        let pattern = format!("{prefix}%");
+        let prefix = prefix.to_string();
         tokio::task::spawn_blocking(move || {
             let conn = conn.lock().unwrap();
-            let mut stmt = conn.prepare_cached("SELECT key FROM kv WHERE key LIKE ?1 ORDER BY key")?;
-            let keys = stmt
-                .query_map(params![pattern], |row| row.get(0))?
-                .collect::<Result<Vec<String>, _>>()?;
+            if prefix.is_empty() {
+                let mut stmt = conn.prepare_cached("SELECT key FROM kv ORDER BY key")?;
+                let keys = stmt
+                    .query_map([], |row| row.get(0))?
+                    .collect::<Result<Vec<String>, _>>()?;
+                return Ok(keys);
+            }
+            // Compute exclusive upper bound: increment last non-0xFF byte
+            let upper = {
+                let mut bytes = prefix.as_bytes().to_vec();
+                let mut result = None;
+                while let Some(last) = bytes.pop() {
+                    if last < 0xFF {
+                        bytes.push(last + 1);
+                        result = Some(String::from_utf8_lossy(&bytes).into_owned());
+                        break;
+                    }
+                }
+                result
+            };
+            let keys = match &upper {
+                Some(end) => {
+                    let mut stmt = conn.prepare_cached(
+                        "SELECT key FROM kv WHERE key >= ?1 AND key < ?2 ORDER BY key",
+                    )?;
+                    stmt.query_map(params![prefix, end], |row| row.get(0))?
+                        .collect::<Result<Vec<String>, _>>()?
+                }
+                None => {
+                    let mut stmt = conn.prepare_cached(
+                        "SELECT key FROM kv WHERE key >= ?1 ORDER BY key",
+                    )?;
+                    stmt.query_map(params![prefix], |row| row.get(0))?
+                        .collect::<Result<Vec<String>, _>>()?
+                }
+            };
             Ok(keys)
         })
         .await
@@ -120,6 +152,7 @@ impl Db {
         let conn = self.conn.clone();
         tokio::task::spawn_blocking(move || {
             let conn = conn.lock().unwrap();
+            conn.execute_batch("BEGIN")?;
             let mut stmt = conn.prepare_cached(
                 "INSERT INTO kv (key, volumes, size) VALUES (?1, ?2, ?3)
                  ON CONFLICT(key) DO UPDATE SET volumes = ?2, size = ?3",
@@ -127,6 +160,8 @@ impl Db {
             for (key, volumes, size) in &records {
                 stmt.execute(params![key, encode_volumes(volumes), size])?;
             }
+            drop(stmt);
+            conn.execute_batch("COMMIT")?;
             Ok(())
         })
         .await
diff --git a/src/lib.rs b/src/lib.rs
index fa5c1fa..4e80a27 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,6 +7,8 @@ pub mod rebuild;
 
 use std::sync::Arc;
 
+const DEFAULT_BODY_LIMIT: usize = 256 * 1024 * 1024; // 256 MB
+
 pub struct Args {
     pub db_path: String,
     pub volumes: Vec<String>,
@@ -14,6 +16,15 @@ pub struct Args {
 }
 
 pub fn build_app(args: &Args) -> axum::Router {
+    if args.replicas > args.volumes.len() {
+        eprintln!(
+            "Error: replication factor ({}) exceeds number of volumes ({})",
+            args.replicas,
+            args.volumes.len()
+        );
+        std::process::exit(1);
+    }
+
     if let Some(parent) = std::path::Path::new(&args.db_path).parent() {
         std::fs::create_dir_all(parent).unwrap_or_else(|e| {
             eprintln!("Failed to create database directory: {e}");
@@ -37,5 +48,6 @@ pub fn build_app(args: &Args) -> axum::Router {
                 .delete(server::delete_key)
                 .head(server::head_key),
         )
+        .layer(axum::extract::DefaultBodyLimit::max(DEFAULT_BODY_LIMIT))
         .with_state(state)
 }