Storage access patterns in HLD - System Design Guide

Problem Statement

When a system stores and retrieves data inefficiently, it causes slow response times and high resource use. Poor access patterns can lead to bottlenecks, increased latency, and wasted storage or compute resources, especially at scale.

Solution

Storage access patterns organize how data is read and written to storage to optimize speed, reduce latency, and improve resource use. By choosing the right pattern—like sequential reads, random access, or caching—the system matches data usage with storage capabilities, improving overall performance.

Architecture

┌───────────────┐       ┌───────────────┐       ┌───────────────┐
│   Application │──────▶│ Storage Access│──────▶│    Storage    │
│   Layer       │       │   Patterns    │       │   Systems     │
└───────────────┘       └───────────────┘       └───────────────┘
        │                      │                       │
        │                      │                       │
        ▼                      ▼                       ▼
  User Requests          Access Pattern          Physical Storage
                         Decision Logic           (Disk, SSD, DB)

This diagram shows how the application layer interacts with storage systems through a storage access pattern layer that decides the best way to read or write data.

Trade-offs

✓ Pros

→

Improves read/write performance by matching data access to storage capabilities.

→

Reduces latency by optimizing data retrieval paths.

→

Enhances scalability by preventing storage bottlenecks.

→

Can lower costs by minimizing unnecessary data movement.

✗ Cons

→

Adds complexity to system design and implementation.

→

Requires understanding of data usage patterns which may change over time.

→

Improper pattern choice can worsen performance or increase costs.

Use when your system handles large volumes of data with varied access needs, such as millions of reads/writes per second or when latency under 100ms is critical.

Avoid when data access is simple and low volume (under 1000 ops/sec) or when system complexity must be minimal.

Real World Examples

Netflix

Uses sequential read patterns for streaming video chunks to optimize bandwidth and reduce latency.

Amazon

Applies caching and random access patterns in DynamoDB to speed up product catalog queries.

Google

Employs columnar storage access patterns in BigQuery to efficiently scan large datasets for analytics.

Alternatives

Caching

Stores frequently accessed data in fast storage to reduce repeated reads from slower storage.

Use when: Choose when read latency is critical and data has high reuse.

Sharding

Splits data horizontally across multiple storage nodes to distribute load.

Use when: Choose when data volume or request load exceeds single node capacity.

Batch Processing

Processes data in large groups at scheduled times instead of real-time access.

Use when: Choose when real-time access is not required and throughput is prioritized.

Summary

Storage access patterns organize how data is read and written to improve system performance.

Choosing the right pattern depends on data volume, latency needs, and workload characteristics.

Common patterns include sequential access, random access, caching, and sharding.

Practice

1. Which storage access pattern reads data in the order it is stored, one piece after another?

easy

A. Sequential access

B. Random access

C. Batch access

D. Streaming access

Solution

1. Step 1: Understand sequential access meaning

   Sequential access means reading data in the exact order it is stored, one after another.

2. Step 2: Compare with other patterns

   Random access reads data in any order, batch access reads groups of data, streaming processes data continuously.

3. Final Answer:

   Sequential access -> Option A

4. Quick Check:

   Sequential = ordered reading [OK]

Hint: Sequential means reading data in stored order [OK]

Common Mistakes:

• Confusing random access with sequential
• Thinking batch means single item reading
• Mixing streaming with batch processing

2. Which of the following is the correct description of random access in storage systems?

easy

A. Data is read in fixed-size batches only

B. Data is accessed only once and then discarded

C. Data is streamed continuously without breaks

D. Data can be read or written at any location without following order

Solution

1. Step 1: Define random access

   Random access allows reading or writing data at any position without following a sequence.

2. Step 2: Eliminate incorrect options

   Batch access reads fixed-size groups, streaming is continuous flow, and discarding after one access is not random access.

3. Final Answer:

   Data can be read or written at any location without following order -> Option D

4. Quick Check:

   Random access = any location access [OK]

Hint: Random means any location, no order needed [OK]

Common Mistakes:

• Confusing batch with random access
• Thinking streaming means random access
• Assuming random access is sequential

3. Consider a system that processes logs by reading 1000 entries at once, then writes results back. Which access pattern best describes this?

medium

A. Random access, because entries are accessed individually

B. Streaming access, because data flows continuously

C. Batch access, because data is processed in groups

D. Sequential access, because logs are read in order

Solution

1. Step 1: Identify processing style

   The system reads 1000 entries at once, meaning it processes data in groups or batches.

2. Step 2: Match with access patterns

   Batch access means handling data in fixed-size groups, streaming is continuous single flow, random is unordered, sequential is one by one.

3. Final Answer:

   Batch access, because data is processed in groups -> Option C

4. Quick Check:

   Batch = group processing [OK]

Hint: Batch means processing data in groups [OK]

Common Mistakes:

• Confusing batch with streaming
• Assuming sequential means batch
• Thinking random access fits batch processing

4. A developer implemented streaming access but notices high latency and data loss. What is the most likely cause?

medium

A. Not handling backpressure in the streaming pipeline

B. Reading data sequentially instead of randomly

C. Using batch processing instead of continuous streaming

D. Accessing data in random order causing delays

Solution

1. Step 1: Understand streaming issues

   Streaming requires managing data flow to avoid overload, called backpressure handling.

2. Step 2: Identify cause of latency and loss

   If backpressure is not handled, data can pile up causing delays and loss.

3. Final Answer:

   Not handling backpressure in the streaming pipeline -> Option A

4. Quick Check:

   Streaming needs backpressure control [OK]

Hint: Streaming needs backpressure handling to avoid loss [OK]

Common Mistakes:

• Confusing batch with streaming
• Ignoring backpressure importance
• Assuming random access causes streaming issues

5. You design a video streaming service that must deliver smooth playback and allow users to jump to any timestamp instantly. Which storage access pattern combination is best?

hard

A. Sequential access for playback and batch access for buffering

B. Streaming access for playback and random access for seeking

C. Batch access for playback and sequential access for seeking

D. Random access for playback and streaming access for buffering

Solution

1. Step 1: Analyze playback needs

   Smooth playback requires continuous data flow, which is streaming access.

2. Step 2: Analyze seeking needs

   Jumping to any timestamp instantly requires random access to data at any position.

3. Step 3: Combine patterns for best design

   Use streaming for continuous playback and random access for quick seeking.

4. Final Answer:

   Streaming access for playback and random access for seeking -> Option B

5. Quick Check:

   Playback = streaming, seeking = random [OK]

Hint: Streaming for play, random for jump [OK]

Common Mistakes:

• Using sequential for seeking which is slow
• Confusing batch with streaming for playback
• Ignoring random access need for seeking