Distributed file systems in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a distributed file system?

A distributed file system is a way to store and access files across multiple computers as if they were on a single machine. It helps share data easily and reliably over a network.

beginner

Why do distributed file systems use replication?

Replication means keeping copies of files on different machines. It helps protect data if one machine fails and improves speed by letting users access the closest copy.

intermediate

What is the role of a metadata server in a distributed file system?

The metadata server keeps track of where files are stored, their names, and permissions. It helps find files quickly without searching all machines.

advanced

How does a distributed file system handle concurrent writes to the same file?

It uses locking or version control to make sure changes don’t conflict. This keeps data consistent and prevents errors when many users write at once.

intermediate

Name two popular distributed file systems and a key feature of each.

HDFS (Hadoop Distributed File System) is designed for big data and stores large files across many machines. Ceph is highly scalable and supports block, object, and file storage.

What is the main purpose of a distributed file system?

A To compress files to save space

B To backup files on a single machine

C To encrypt files for security

D To store files across multiple machines for easy access

Distributed file systems store files on many machines to allow easy and reliable access over a network.

Which component tracks file locations in a distributed file system?

A Data node

B Metadata server

C Client

D Load balancer

The metadata server keeps information about where files are stored and their details.

Why is replication important in distributed file systems?

A To improve fault tolerance and speed

B To reduce file size

C To encrypt data

D To limit user access

Replication creates copies of data to protect against failures and improve access speed.

How do distributed file systems maintain consistency during concurrent writes?

A By locking files or using version control

B By ignoring conflicts

C By deleting old files

D By copying files to clients

Locking or version control prevents conflicting changes and keeps data consistent.

Which distributed file system is designed for big data storage?

A Ceph

B NFS

C HDFS

D FTP

HDFS is built to store and process large data sets across many machines.

Explain how a distributed file system ensures data availability and fault tolerance.

Think about how copies help when one machine fails.

Describe the role of metadata servers and data nodes in a distributed file system.

Consider who knows where files are and who holds the files.

Practice

1. What is the main role of a metadata server in a distributed file system?

easy

A. To keep track of file locations and directory structure

B. To store the actual file data

C. To provide network connectivity between clients

D. To encrypt files before storage

Solution

1. Step 1: Understand metadata server function

   The metadata server manages information about where files are stored and their organization.

2. Step 2: Differentiate from storage nodes

   Storage nodes hold the actual file data, not metadata servers.

3. Final Answer:

   To keep track of file locations and directory structure -> Option A

4. Quick Check:

   Metadata server = file info tracker [OK]

Hint: Metadata servers track file info, not data storage [OK]

Common Mistakes:

• Confusing metadata server with storage node
• Thinking metadata server stores file data
• Assuming metadata server handles encryption

2. Which of the following is the correct sequence when a client reads a file in a distributed file system?

easy

A. Client contacts only the storage node

B. Client contacts storage node, then metadata server

C. Client contacts only the metadata server

D. Client contacts metadata server, then storage node

Solution

1. Step 1: Identify client interaction flow

   The client first asks the metadata server for file location details.

2. Step 2: Access data from storage node

   After getting location info, the client reads the actual file data from the storage node.

3. Final Answer:

   Client contacts metadata server, then storage node -> Option D

4. Quick Check:

   Read flow = metadata server then storage node [OK]

Hint: Client asks metadata first, then reads data [OK]

Common Mistakes:

• Reversing the order of contacting servers
• Assuming client contacts only one server type
• Ignoring metadata server role in locating files

3. Consider a distributed file system where a file is split into 3 chunks stored on different storage nodes. If the metadata server returns chunk locations as [Node1, Node2, Node3], what happens when the client requests the file?

medium

A. Client reads all chunks sequentially from Node1 only

B. Client reads chunks in parallel from Node1, Node2, and Node3

C. Client reads chunks randomly from any node

D. Client reads only the first chunk from Node1

Solution

1. Step 1: Understand chunk distribution

   The file is divided into chunks stored on separate nodes for parallel access.

2. Step 2: Client reads chunks efficiently

   To speed up reading, the client fetches all chunks at the same time from their respective nodes.

3. Final Answer:

   Client reads chunks in parallel from Node1, Node2, and Node3 -> Option B

4. Quick Check:

   Parallel chunk reads = faster file access [OK]

Hint: Chunks read in parallel from all nodes [OK]

Common Mistakes:

• Assuming client reads all chunks from one node
• Thinking chunks are read randomly
• Ignoring parallelism benefits

4. A distributed file system client tries to read a file but gets an error because the metadata server is unreachable. What is the most likely cause?

medium

A. Client has no network access to metadata server

B. Storage nodes are down

C. File data is corrupted on storage nodes

D. Client requested a non-existent file chunk

Solution

1. Step 1: Identify error source

   The error is due to inability to reach the metadata server, not storage nodes.

2. Step 2: Analyze network access

   Most likely, the client cannot connect to the metadata server because of network issues or firewall rules.

3. Final Answer:

   Client has no network access to metadata server -> Option A

4. Quick Check:

   Metadata server unreachable = network issue [OK]

Hint: Metadata server unreachable means network problem [OK]

Common Mistakes:

• Blaming storage nodes for metadata server errors
• Assuming file corruption causes metadata errors
• Confusing chunk existence with server reachability

5. You are designing a distributed file system for a company with many users reading large files frequently but rarely writing. Which design choice improves read performance the most?

hard

A. Disable caching to ensure fresh data on every read

B. Store all file chunks on a single storage node to simplify access

C. Use multiple metadata servers with caching to reduce lookup latency

D. Limit the number of storage nodes to reduce network overhead

Solution

1. Step 1: Analyze workload pattern

   Many reads and few writes mean caching metadata can speed up file location lookups.

2. Step 2: Evaluate design options

   Multiple metadata servers with caching reduce latency and avoid bottlenecks, improving read speed.

3. Step 3: Reject other options

   Storing all chunks on one node limits scalability; disabling caching slows reads; fewer nodes reduce parallelism.

4. Final Answer:

   Use multiple metadata servers with caching to reduce lookup latency -> Option C

5. Quick Check:

   Metadata caching + multiple servers = faster reads [OK]

Hint: Cache metadata and use multiple servers for fast reads [OK]

Common Mistakes:

• Putting all data on one node limits scalability
• Disabling caching hurts read performance
• Reducing nodes lowers parallel access