diff --git a/lib/dat/reader.ts b/lib/dat/reader.ts
index f91e889..c0c4706 100644
--- a/lib/dat/reader.ts
+++ b/lib/dat/reader.ts
@@ -34,12 +34,26 @@ export class DataReader<T> {
         this.indexReader.open(this.indexPath);
     }
 
-    private readRecord(offset: bigint, length: number): Buffer {
-        if (this.fd === null) throw new Error('Data file not opened');
-
+    private readRecordAt(fd: number, offset: bigint, length: number): { data: T; length: number } | null {
         const buf = Buffer.alloc(length);
-        fs.readSync(this.fd, buf, 0, length, Number(offset));
-        return buf;
+        fs.readSync(fd, buf, 0, length, Number(offset));
+        const result = DataProtocol.deserializeRecord(
+            buf,
+            0,
+            this.serializer
+        );
+        return result;
+    }
+
+    private readNextRecord(fd: number, length: number): { data: T; length: number } | null {
+        const buf = Buffer.alloc(length);
+        fs.readSync(fd, buf, 0, length, null);
+        const result = DataProtocol.deserializeRecord(
+            buf,
+            0,
+            this.serializer
+        );
+        return result;
     }
 
     getHeader(): DataHeader {
@@ -53,15 +67,12 @@ export class DataReader<T> {
         const found = this.indexReader.binarySearchBySequence(sequence);
         if (!found) return null;
 
-        const buf = this.readRecord(found.entry.offset, found.entry.length);
-        const result = DataProtocol.deserializeRecord(
-            buf,
-            0,
-            this.serializer
-        );
+        const result = this.readRecordAt(this.fd, found.entry.offset, found.entry.length);
+
         if (!result) return null;
 
         return {
+            index: found.index,
             sequence: found.entry.sequence,
             timestamp: found.entry.timestamp,
             data: result.data,
@@ -74,112 +85,93 @@ export class DataReader<T> {
         const entry = this.indexReader.getEntry(index);
         if (!entry) return null;
 
-        const buf = this.readRecord(entry.offset, entry.length);
-        const result = DataProtocol.deserializeRecord(
-            buf,
-            0,
-            this.serializer
-        );
+        const result = this.readRecordAt(this.fd, entry.offset, entry.length);
+
         if (!result) return null;
 
         return {
+            index: index,
             sequence: entry.sequence,
             timestamp: entry.timestamp,
             data: result.data,
         };
     }
 
-    getBulkData(startSeq: number, endSeq: number): DataEntry<T>[] {
+    getBulkDataBySequence(startSeq: number, endSeq: number): DataEntry<T>[] {
         if (this.fd === null) throw new Error('Data file not opened');
 
         const results: DataEntry<T>[] = [];
-        const indexHeader = this.indexReader.getHeader();
-        let startIdx = this.findStartIndex(startSeq, indexHeader.writtenCnt);
+        const indexResults = this.indexReader.findBySequenceRange(startSeq, endSeq);
+        if (indexResults.length === 0) return [];
 
-        for (let i = startIdx; i < indexHeader.writtenCnt; i++) {
-            const entry = this.indexReader.getEntry(i);
-            if (!entry) continue;
+        const firstRecord = this.readRecordAt(this.fd, indexResults[0].entry.offset, indexResults[0].entry.length);
+        if (firstRecord) {
+            results.push({
+                index: indexResults[0].index,
+                sequence: indexResults[0].entry.sequence,
+                timestamp: indexResults[0].entry.timestamp,
+                data: firstRecord.data,
+            })
+        }
 
-            if (entry.sequence > endSeq) break;
+        for (let i = 1; i < indexResults.length; i++) {
+            const { index, entry } = indexResults[i];
+            const result = this.readNextRecord(this.fd, entry.length);
+            if (!result) continue;
 
-            if (entry.sequence >= startSeq) {
-                const buf = this.readRecord(entry.offset, entry.length);
-                const result = DataProtocol.deserializeRecord(
-                    buf,
-                    0,
-                    this.serializer
-                );
-                if (result) {
-                    results.push({
-                        sequence: entry.sequence,
-                        timestamp: entry.timestamp,
-                        data: result.data,
-                    });
-                }
-            }
+            results.push({
+                index: index,
+                sequence: entry.sequence,
+                timestamp: entry.timestamp,
+                data: result.data,
+            });
         }
 
         return results;
     }
 
-    private findStartIndex(targetSeq: number, writtenCnt: number): number {
-        let left = 0;
-        let right = writtenCnt - 1;
-        let result = 0;
-
-        while (left <= right) {
-            const mid = Math.floor((left + right) / 2);
-            const entry = this.indexReader.getEntry(mid);
-
-            if (!entry) {
-                right = mid - 1;
-                continue;
-            }
-
-            if (entry.sequence >= targetSeq) {
-                result = mid;
-                right = mid - 1;
-            } else {
-                left = mid + 1;
-            }
-        }
-
-        return result;
-    }
-
     getBulkDataByTime(startTs: bigint, endTs: bigint): DataEntry<T>[] {
         if (this.fd === null) throw new Error('Data file not opened');
 
-        const indexResults = this.indexReader.findByTimeRange(startTs, endTs);
         const results: DataEntry<T>[] = [];
+        const indexResults = this.indexReader.findByTimeRange(startTs, endTs);
+        if (indexResults.length === 0) return [];
 
-        for (const { entry } of indexResults) {
-            const buf = this.readRecord(entry.offset, entry.length);
-            const result = DataProtocol.deserializeRecord(
-                buf,
-                0,
-                this.serializer
-            );
-            if (result) {
-                results.push({
-                    sequence: entry.sequence,
-                    timestamp: entry.timestamp,
-                    data: result.data,
-                });
-            }
+        const firstRecord = this.readRecordAt(this.fd, indexResults[0].entry.offset, indexResults[0].entry.length);
+        if (firstRecord) {
+            results.push({
+                index: indexResults[0].index,
+                sequence: indexResults[0].entry.sequence,
+                timestamp: indexResults[0].entry.timestamp,
+                data: firstRecord.data,
+            })
+        }
+
+        for (let i = 1; i < indexResults.length; i++) {
+            const { index, entry } = indexResults[i];
+            const result = this.readNextRecord(this.fd, entry.length);
+            if (!result) continue;
+
+            results.push({
+                index: index,
+                sequence: entry.sequence,
+                timestamp: entry.timestamp,
+                data: result.data,
+            });
         }
 
         return results;
     }
 
+    /*
     getAllData(): DataEntry<T>[] {
         if (this.fd === null) throw new Error('Data file not opened');
-
+     
         const entries = this.indexReader.getAllEntries();
         const results: DataEntry<T>[] = [];
-
+     
         for (const entry of entries) {
-            const buf = this.readRecord(entry.offset, entry.length);
+            const buf = this.readRecordAt(this.fd, entry.offset, entry.length);
             const result = DataProtocol.deserializeRecord(
                 buf,
                 0,
@@ -193,9 +185,10 @@ export class DataReader<T> {
                 });
             }
         }
-
+     
         return results;
     }
+    */
 
     getRecordCount(): number {
         return this.indexReader.getHeader().writtenCnt;
@@ -209,6 +202,32 @@ export class DataReader<T> {
         return this.indexReader.getFlags();
     }
 
+    readNext(): DataEntry<T> | null {
+        if (this.fd === null) throw new Error('Data file not opened');
+
+        const next = this.indexReader.getNextEntry();
+        if (!next) return null;
+        const { index, entry } = next;
+
+        const result = this.readNextRecord(this.fd, entry.length);
+        if (!result) return null;
+
+        return {
+            index,
+            sequence: entry.sequence,
+            timestamp: entry.timestamp,
+            data: result.data,
+        };
+    }
+
+    getCurrentIndex(): number {
+        return this.indexReader.getCurrentIndex();
+    }
+
+    getCurrentSequence(): number {
+        return this.indexReader.getCurrentSequence();
+    }
+
     close(): void {
         if (this.fd !== null) {
             fs.closeSync(this.fd);
diff --git a/lib/dat/types.ts b/lib/dat/types.ts
index 7041b99..880bd53 100644
--- a/lib/dat/types.ts
+++ b/lib/dat/types.ts
@@ -7,6 +7,7 @@ export interface Serializer<T> {
 }
 
 export interface DataEntry<T> {
+    index: number;
     sequence: number;
     timestamp: bigint;
     data: T;
diff --git a/lib/idx/reader.ts b/lib/idx/reader.ts
index 653348c..b747699 100644
--- a/lib/idx/reader.ts
+++ b/lib/idx/reader.ts
@@ -1,22 +1,36 @@
 // src/index-file/reader.ts
 import * as fs from 'node:fs';
+import { INDEX_HEADER_SIZE, INDEX_ENTRY_SIZE, FLAG_VALID } from './constants.js';
 import { IndexProtocol } from './protocol.js';
 import type { IndexHeader, IndexEntry } from './types.js';
 
 export class IndexReader {
-    private buffer: Buffer | null = null;
+    private fd: number | null = null;
     private header: IndexHeader | null = null;
 
     private path: string | null = null;
 
+    // Sequential read state
+    private currentIndex: number = 0;
+    private currentSequence: number = 0;
+
+    // Reusable buffer for reading entries
+    private entryBuffer: Buffer = Buffer.alloc(INDEX_ENTRY_SIZE);
+
     constructor() {
     }
 
     open(idxFilePath: string): void {
-        // Read entire file into buffer (simpler than mmap for read-only access)
         this.path = idxFilePath;
-        this.buffer = fs.readFileSync(this.path);
-        this.header = IndexProtocol.readHeader(this.buffer);
+        this.fd = fs.openSync(this.path, 'r');
+
+        // Read header only (64 bytes)
+        const headerBuf = Buffer.alloc(INDEX_HEADER_SIZE);
+        fs.readSync(this.fd, headerBuf, 0, INDEX_HEADER_SIZE, 0);
+        this.header = IndexProtocol.readHeader(headerBuf);
+        // 아예 최초에는 -1 로 두어서 readNextEntry 할때 자동 ++ 할때 오류를 검증
+        this.currentIndex = -1;
+        this.currentSequence = -1;
     }
 
     getHeader(): IndexHeader {
@@ -30,90 +44,244 @@ export class IndexReader {
     }
 
     getEntry(index: number): IndexEntry | null {
-        if (!this.buffer || !this.header) throw new Error('Index file not opened');
-        if (index < 0 || index >= this.header.entryCount) return null;
-        return IndexProtocol.readEntry(this.buffer, index);
-    }
-
-    findBySequence(sequence: number): { index: number; entry: IndexEntry } | null {
-        if (!this.buffer || !this.header) throw new Error('Index file not opened');
-
-        for (let i = 0; i < this.header.writtenCnt; i++) {
-            const entry = IndexProtocol.readEntry(this.buffer, i);
-            if (entry && entry.sequence === sequence) {
-                return { index: i, entry };
-            }
-        }
-        return null;
+        if (this.fd === null || !this.header) throw new Error('Index file not opened');
+        if (index < 0 || index >= this.header.writtenCnt) return null;
+        return this.readEntryAt(this.fd, index);
     }
 
     findBySequenceRange(startSeq: number, endSeq: number): { index: number; entry: IndexEntry }[] {
-        if (!this.buffer || !this.header) throw new Error('Index file not opened');
+        if (this.fd === null || !this.header) throw new Error('Index file not opened');
 
         const results: { index: number; entry: IndexEntry }[] = [];
-        for (let i = 0; i < this.header.writtenCnt; i++) {
-            const entry = IndexProtocol.readEntry(this.buffer, i);
-            if (entry && entry.sequence >= startSeq && entry.sequence <= endSeq) {
-                results.push({ index: i, entry });
-            }
+        const first = this.searchSequenceLowerBound(this.fd, this.header.writtenCnt, startSeq);
+        if (first == null) return [];
+
+        results.push(first);
+        for (let i = this.currentIndex; i < this.header.writtenCnt; i++) {
+            const entry = this.readNextEntry(this.fd);
+            if (entry == null || entry.sequence > endSeq) break;
+            results.push({ index: this.currentIndex, entry });
         }
         return results;
     }
 
+    /*
     getAllEntries(): IndexEntry[] {
-        if (!this.buffer || !this.header) throw new Error('Index file not opened');
+        if (this.fd === null || !this.header) throw new Error('Index file not opened');
 
         const entries: IndexEntry[] = [];
         for (let i = 0; i < this.header.writtenCnt; i++) {
-            const entry = IndexProtocol.readEntry(this.buffer, i);
+            const entry = this.readEntryAt(this.fd, i);
             if (entry) entries.push(entry);
         }
         return entries;
     }
+    */
 
     findByTimeRange(startTs: bigint, endTs: bigint): { index: number; entry: IndexEntry }[] {
-        if (!this.buffer || !this.header) throw new Error('Index file not opened');
+        if (this.fd === null || !this.header) throw new Error('Index file not opened');
 
         const results: { index: number; entry: IndexEntry }[] = [];
-        for (let i = 0; i < this.header.writtenCnt; i++) {
-            const entry = IndexProtocol.readEntry(this.buffer, i);
-            if (entry && entry.timestamp >= startTs && entry.timestamp <= endTs) {
-                results.push({ index: i, entry });
-            }
+        const first = this.searchTimestampLowerBound(this.fd, this.header.writtenCnt, startTs);
+        if (first === null) return [];
+
+        results.push(first);
+        for (let i = this.currentIndex; i < this.header.writtenCnt; i++) {
+            const entry = this.readNextEntry(this.fd);
+            if (entry == null || entry.timestamp > endTs) break;
+            results.push({ index: this.currentIndex, entry });
         }
         return results;
     }
 
     binarySearchBySequence(targetSeq: number): { index: number; entry: IndexEntry } | null {
-        if (!this.buffer || !this.header) throw new Error('Index file not opened');
+        if (this.fd === null || !this.header) throw new Error('Index file not opened');
+        return this.searchSequenceExact(this.fd, this.header.writtenCnt, targetSeq);
+    }
 
+    getNextEntry(): { index: number; entry: IndexEntry } | null {
+        if (this.fd === null || !this.header) throw new Error('Index file not opened');
+
+        if (this.currentIndex >= (this.header.writtenCnt - 1)) {
+            return null;
+        }
+
+        const entry = this.readNextEntry(this.fd);
+        if (!entry) return null;
+
+        return { index: this.currentIndex, entry };
+    }
+
+    getCurrentIndex(): number {
+        return this.currentIndex;
+    }
+
+    getCurrentSequence(): number {
+        return this.currentSequence;
+    }
+
+    close(): void {
+        if (this.fd !== null) {
+            fs.closeSync(this.fd);
+            this.fd = null;
+        }
+        this.header = null;
+        this.currentIndex = -1;
+        this.currentSequence = -1;
+    }
+
+    // ######################################################################
+    //                        Private methods
+    // ######################################################################
+    private readEntryAt(fd: number, index: number): IndexEntry | null {
+        const offset = INDEX_HEADER_SIZE + index * INDEX_ENTRY_SIZE;
+        fs.readSync(fd, this.entryBuffer, 0, INDEX_ENTRY_SIZE, offset);
+
+        const flags = this.entryBuffer.readUInt32LE(24);
+        const indexEntry = this.buildIndexEntry(flags);
+
+        this.currentIndex = index;
+        this.currentSequence = indexEntry.sequence;
+
+        if (!(flags & FLAG_VALID)) return null;
+        return indexEntry;
+    }
+
+    private readNextEntry(fd: number): IndexEntry | null {
+        fs.readSync(fd, this.entryBuffer, 0, INDEX_ENTRY_SIZE, null);
+
+        const flags = this.entryBuffer.readUInt32LE(24);
+        const indexEntry = this.buildIndexEntry(flags);
+
+        ++this.currentIndex;
+        this.currentSequence = indexEntry.sequence;
+
+        if (!(flags & FLAG_VALID)) return null;
+        return indexEntry;
+    }
+
+    private buildIndexEntry(flags: number): IndexEntry {
+        return {
+            sequence: this.entryBuffer.readUInt32LE(0),
+            timestamp: this.entryBuffer.readBigUInt64LE(4),
+            offset: this.entryBuffer.readBigUInt64LE(12),
+            length: this.entryBuffer.readUInt32LE(20),
+            flags,
+            checksum: this.entryBuffer.readUInt32LE(28),
+        };
+    }
+
+    // Timestamp binary search methods
+    private searchTimestampExact(fd: number, writtenCnt: number, targetTs: bigint): { index: number; entry: IndexEntry } | null {
         let left = 0;
-        let right = this.header.writtenCnt - 1;
+        let right = writtenCnt - 1;
 
         while (left <= right) {
             const mid = Math.floor((left + right) / 2);
-            const entry = IndexProtocol.readEntry(this.buffer, mid);
+            const entry = this.readEntryAt(fd, mid);
+            if (!entry) { right = mid - 1; continue; }
 
-            if (!entry) {
+            if (entry.timestamp === targetTs) return { index: mid, entry };
+            if (entry.timestamp < targetTs) left = mid + 1;
+            else right = mid - 1;
+        }
+        return null;
+    }
+
+    private searchTimestampLowerBound(fd: number, writtenCnt: number, targetTs: bigint): { index: number; entry: IndexEntry } | null {
+        let left = 0;
+        let right = writtenCnt - 1;
+        let result: { index: number; entry: IndexEntry } | null = null;
+
+        while (left <= right) {
+            const mid = Math.floor((left + right) / 2);
+            const entry = this.readEntryAt(fd, mid);
+            if (!entry) { right = mid - 1; continue; }
+
+            if (entry.timestamp >= targetTs) {
+                result = { index: mid, entry };
                 right = mid - 1;
-                continue;
+            } else {
+                left = mid + 1;
             }
+        }
+        return result;
+    }
 
-            if (entry.sequence === targetSeq) {
-                return { index: mid, entry };
-            } else if (entry.sequence < targetSeq) {
+    private searchTimestampUpperBound(fd: number, writtenCnt: number, targetTs: bigint): { index: number; entry: IndexEntry } | null {
+        let left = 0;
+        let right = writtenCnt - 1;
+        let result: { index: number; entry: IndexEntry } | null = null;
+
+        while (left <= right) {
+            const mid = Math.floor((left + right) / 2);
+            const entry = this.readEntryAt(fd, mid);
+            if (!entry) { right = mid - 1; continue; }
+
+            if (entry.timestamp <= targetTs) {
+                result = { index: mid, entry };
                 left = mid + 1;
             } else {
                 right = mid - 1;
             }
         }
+        return result;
+    }
 
+    // Sequence binary search methods
+    private searchSequenceExact(fd: number, writtenCnt: number, targetSeq: number): { index: number; entry: IndexEntry } | null {
+        let left = 0;
+        let right = writtenCnt - 1;
+
+        while (left <= right) {
+            const mid = Math.floor((left + right) / 2);
+            const entry = this.readEntryAt(fd, mid);
+            if (!entry) { right = mid - 1; continue; }
+
+            if (entry.sequence === targetSeq) return { index: mid, entry };
+            if (entry.sequence < targetSeq) left = mid + 1;
+            else right = mid - 1;
+        }
         return null;
     }
 
-    close(): void {
-        // Simply release buffer reference (GC will handle cleanup)
-        this.buffer = null;
-        this.header = null;
+    private searchSequenceLowerBound(fd: number, writtenCnt: number, targetSeq: number): { index: number; entry: IndexEntry } | null {
+        let left = 0;
+        let right = writtenCnt - 1;
+        let result: { index: number; entry: IndexEntry } | null = null;
+
+        while (left <= right) {
+            const mid = Math.floor((left + right) / 2);
+            const entry = this.readEntryAt(fd, mid);
+            if (!entry) { right = mid - 1; continue; }
+
+            if (entry.sequence >= targetSeq) {
+                result = { index: mid, entry };
+                right = mid - 1;
+            } else {
+                left = mid + 1;
+            }
+        }
+        return result;
+    }
+
+    private searchSequenceUpperBound(fd: number, writtenCnt: number, targetSeq: number): { index: number; entry: IndexEntry } | null {
+        let left = 0;
+        let right = writtenCnt - 1;
+        let result: { index: number; entry: IndexEntry } | null = null;
+
+        while (left <= right) {
+            const mid = Math.floor((left + right) / 2);
+            const entry = this.readEntryAt(fd, mid);
+            if (!entry) { right = mid - 1; continue; }
+
+            if (entry.sequence <= targetSeq) {
+                result = { index: mid, entry };
+                left = mid + 1;
+            } else {
+                right = mid - 1;
+            }
+        }
+        return result;
     }
 }