P4L2 - Distributed File Systems 01

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Questions and Answers

What type of interfaces do distributed storage facilities need to export?

Dynamic
Random
Well-defined (correct)
Unstable

Which aspect helps managers determine the inventories and delivery times accurately in distributed storage facilities?

Constantly updating inventories (correct)
Not exporting interfaces
Having limited resources
Randomly changing delivery schedules

What is a characteristic of some distributed storage facilities that provide both storage and processing services?

Only offer storage
Replicate files across different machines
Export unstable interfaces
Specialized for different types of toys or parts (correct)

How do distributed file systems ensure consistency when files are updated by clients?

By tracking file update information (A) Signup and view all the answers

In which situation is a replica of a file maintained on other server machines in a distributed file system?

In case of failures (A) Signup and view all the answers

What type of interface allows operating systems to utilize multiple types of storage devices regardless of location?

Virtual File System (VFS) (B) Signup and view all the answers

How are files typically stored in a distributed file system where the file server is distributed across multiple machines?

Files are split among different physical machines (D) Signup and view all the answers

'Cache coherence algorithms' mentioned in the text are necessary for what purpose in distributed file systems?

'Cache coherence algorithms' maintain a coherent view of shared data within a cache memory (D) Signup and view all the answers

'DFS' in the context provided stands for:

'Distributed File System' (D) Signup and view all the answers

'VFS' mentioned in the text is an abbreviation for:

'Virtual File System' (D) Signup and view all the answers

What is one of the benefits of load balancing client requests across all replicas?

Better performance (A) Signup and view all the answers

Why is write-update to the file system state more complex when dealing with replicas?

Consistency among all replicas is required (A) Signup and view all the answers

What is one downside of forcing every write-update to every replica?

Slows down all writes (C) Signup and view all the answers

How does partitioning contribute to greater availability in the file system compared to a single server design?

Each server holds fewer files for quicker responses (D) Signup and view all the answers

What technique can be used to resolve differences in the state of a file on different replicas?

Voting where majority wins (D) Signup and view all the answers

Why does the replication model have a limitation in terms of scalability compared to partitioning?

Every server has to hold all the files (C) Signup and view all the answers

What file sharing semantics are typically used in a DFS to maintain acceptable performance?

Periodic updates with time intervals (B) Signup and view all the answers

In a single node Unix environment, why can Process B read 'abc' before the change is pushed to disk?

Memory buffer sharing among processes (D) Signup and view all the answers

What is a limitation of using session semantics in DFS?

Clients can't concurrently share files (B) Signup and view all the answers

Which mechanism in DFS helps correct conflicts periodically?

Periodic updates with time intervals (B) Signup and view all the answers

Why might the buffer cache prove not to be very useful in a scenario with high request-interleaving?

Lack of cache locality (A) Signup and view all the answers

What is the main drawback of using immutable file semantics in a distributed storage system?

Unable to upload new versions of files (B) Signup and view all the answers

How do session semantics handle file updates in DFS when a client closes a file?

Flush all cached changes to the server (B) Signup and view all the answers

Which type of operation allows a client to synchronize its state explicitly with the remote server in a DFS?

"Explicit" synchronization operation (C) Signup and view all the answers

Why might periodic updates with time intervals lead to potential inconsistency in a distributed file system?

"Implicit" state synchronization (A) Signup and view all the answers

"Leases" in DFS are analogous to which familiar concept in regular local file systems?

"File locks" (C) Signup and view all the answers

What is the main reason clients need to interact with the server in the discussed model?

To notify the server of file modifications made by clients. (C) Signup and view all the answers

Why does the text mention that the server is still in the loop in this model?

To highlight the server's consistency maintenance role. (B) Signup and view all the answers

What makes a stateless file server unsuitable for practical models relying on caching?

Prevention of consistency management due to no maintained state. (D) Signup and view all the answers

Why is it mentioned that stateless servers prevent the use of caching?

Because requests have to contain detailed file information. (D) Signup and view all the answers

What is a significant advantage of stateless servers in terms of failure resilience?

Quick recovery from server failures without reissuing requests. (D) Signup and view all the answers

Why does a stateful file server benefit from maintaining information about clients in the system?

To allow for data caching while ensuring consistency. (D) Signup and view all the answers

What role does state play in enabling a file server to provide locking mechanisms?

'Locks' files to ensure exclusive write access by clients. (C) Signup and view all the answers

Why is it stated that stateful servers are capable of supporting incremental operations?

'Locks' on files allow tracking changes and supporting small updates. (D) Signup and view all the answers

Which characteristic distinguishes a stateful server from a stateless server regarding client information?

'Locks' on files for write access control. (A) Signup and view all the answers

What is one significant downside of stateless servers mentioned in the text?

Preclusion of caching, hindering performance optimization (D) Signup and view all the answers

What is one of the challenges of a stateful server in a distributed file system?

Maintaining checkpointed state for consistency (D) Signup and view all the answers

In a stateless design, why must every single request be fully described?

To handle failures and maintain a consistent file system state (D) Signup and view all the answers

What does caching in a distributed file system allow clients to do?

Maintain local copies of portions of files (D) Signup and view all the answers

Why is maintaining cache coherence challenging in a distributed file system?

Because of varying communication costs and latencies (B) Signup and view all the answers

What is an effective way to trigger coherence mechanisms in a distributed file system according to the text?

Periodically or on demand (D) Signup and view all the answers

Where can file caching be done within a distributed file system, based on the text?

Client's memory, local storage, and server-side buffer cache (D) Signup and view all the answers

What problem arises when a client caches a portion of a file locally and another client modifies the same file?

Cache inconsistency between clients (A) Signup and view all the answers

Why might storing cache components on local storage devices benefit clients in a distributed file system?

To minimize latency for remote file access (A) Signup and view all the answers

What does client-driven cache coherence mechanisms entail in a distributed file system?

Client initiating cache update requests from the server (B) Signup and view all the answers

Why is it important for clients to maintain local copies of portions of files in a distributed file system?

To perform operations locally and reduce server load (A) Signup and view all the answers

What type of guarantees could the file system provide in a distributed file system with transactional semantics?

Transactional guarantees (B) Signup and view all the answers

In a server-driven mechanism for file sharing, what does session-semantics imply?

Changes made to files become visible when the file is closed (A) Signup and view all the answers

Why is it essential to understand the access pattern when designing a file system for a specific file sharing semantics?

To determine how often files are shared (C) Signup and view all the answers

What is the common reason for treating regular files and directories differently in a file system design?

Directories are more frequently shared than regular files (A) Signup and view all the answers

What is the purpose of providing transactional guarantees in a distributed file system?

To ensure atomic commit of multiple file operations (C) Signup and view all the answers

How do session semantics impact overlapping sessions in a distributed file system?

Different versions of the same file may be visible (B) Signup and view all the answers

What is the focus of discussion in this lesson?

Distributed file systems (B) Signup and view all the answers

Which research paper is discussed in this lesson?

Caching in the Sprite Network File System (B) Signup and view all the answers

What is the purpose of discussing the Sprite Network File System?

To explain its design decisions (C) Signup and view all the answers

What is a distributed file system likened to in this lesson?

A distributed storage facility (B) Signup and view all the answers

What are the two main requirements for both distributed file systems and storage facilities?

Consistent state and well-defined interface (B) Signup and view all the answers

What distribution models do distributed file systems support?

Both decentralized and hierarchical models (C) Signup and view all the answers

What is a potential downside of the Upload/Download model in a remote file service?

The client needs to download the entire file even for a minor file modification. (B) Signup and view all the answers

What is a benefit of the True Remote File Access model in a remote file service?

It ensures that the server has full control over how clients access and modify files. (B) Signup and view all the answers

What is a limitation of the replicated model in a file system?

It requires every server to store all files, limiting scalability. (C) Signup and view all the answers

What is an advantage of the partitioned model in a file system?

It enables the addition of more machines for scalability. (B) Signup and view all the answers

What is a potential downside of the True Remote File Access model?

It can lead to higher latencies for file operations. (D) Signup and view all the answers

What is a benefit of allowing clients to use their local resources for caching?

It removes load from the server. (D) Signup and view all the answers

What is a limitation of the Upload/Download model in a remote file service?

It can lead to inefficiencies for minor file modifications. (D) Signup and view all the answers

What is a potential benefit of combining replication and partitioning in a file system?

It enables independent replication of each partition. (B) Signup and view all the answers

What is a potential disadvantage of the True Remote File Access model?

It can lead to overloading of the server. (D) Signup and view all the answers

What is a benefit of allowing clients to download some blocks of a file in a remote file service?

It removes load from the server. (C) Signup and view all the answers

Study Notes

Distributed File Systems

A distributed file system is a file system that can be organized in different ways:
- Clients and server on separate machines, with one file server.
- File server distributed on multiple machines, with file replication or partitioning.

Stateful vs Stateless File Server

A stateless file server:
- Does not maintain information about clients or files.
- Each request is self-contained and has all necessary parameters.
- Suitable for extreme file service models (upload-download or true remote file access).
- Cannot be used for caching, which is an important performance optimization technique.
- Benefits: no resources consumed on the server side, resilient design, and easy recovery from failures.
A stateful file server:
- Maintains information about clients and files (e.g., which clients have portions of a file cached).
- Allows data to be cached and guarantees consistency.
- Provides additional benefits, such as locking and incremental operations.
- Requires more complex mechanisms for recovery from failures.

Caching State in a DFS

Caching is a general optimization technique in DFS:
- Clients can locally maintain portions of the state, perform some operations locally, and contact the server only when necessary.
- Caching can be done at the client's memory, local storage devices, or on the server-side in the buffer cache.
- Requires coherence mechanisms to maintain consistency between cached and server-side files.
- Different approaches include write-update, write-invalidate, and client-driven or server-driven coherence mechanisms.

File sharing semantics on a DFS differ from single-node Unix environments:
- Updates may not be immediately visible to other processes due to message latencies.
- DFS may sacrifice some consistency for performance and use relaxed file sharing semantics.
Session semantics:
- Clients write back changes to the server when a file is closed.
- Clients check with the server for updates when a file is opened.
- Provides some consistency guarantees, but may not be suitable for concurrent file sharing.
Periodic updates:
- Clients propagate updates to the server periodically.
- Server notifications and invalidations are also sent periodically.
- Establishes time bounds for potential inconsistencies.

Immutable File Semantics

Immutable file semantics:
- Files cannot be modified once uploaded.
- Suitable for distributed storage facilities.
- Benefits: better performance, availability, and fault tolerance.
- Downsides: more complex write-update operations, and resolving consistency among replicas.

Distributed Storage Facilities

Properties of distributed storage facilities:
- Accessed via well-defined interfaces.
- Update inventories to maintain consistent information.
- Can be configured in different ways (storage-only, storage and processing, specialized for different products).
Relation to distributed file systems:
- Also accessed via high-level interfaces (Virtual File System).
- Maintain consistent state of files among clients.
- Can be implemented via different distribution models (replication, partitioning, or peer-to-peer nodes).### Distributed File Systems (DFS)
DFS needs to provide transactional guarantees, allowing clients to specify a collection of files or operations as a single transaction.
The file system should make guarantees that all changes are atomically committed and atomically visible.

DFS Data Structure

In a server-driven mechanism with session-semantics, the per-file data structure should include:
- Information about current readers
- Information about current writers (potentially multiple)
- Version number to track changes and resolve conflicts

File vs Directory Service

Understanding the workload (access pattern) is crucial for design and optimization considerations.
File systems have different access patterns for regular files and directories, including:
- Locality
- Lifetime of files
- Size distribution
- Access frequency
It is common to treat regular files and directories differently, e.g., adopting different semantics or using periodic updates with less frequent write-backs for regular files.

Replication and Partitioning

The file server can be distributed via replication or partitioning.
Replication involves replicating the file system onto multiple machines, with each machine holding an exact replica of all files.
Partitioning involves dividing files across multiple machines, allowing for easier scaling.

Visual Metaphor

Distributed file systems can be compared to distributed storage facilities, with a focus on maintaining consistent state.
Both replicated and partitioned models can be used, or a combination of both.
Partitioning allows for easier scaling, while replication provides consistency.

Remote File Service Extremes

The Upload/Download model:
- Benefits: fast local operations, easy to implement
- Downsides: entire file needs to be downloaded, loss of file access control
The True Remote File Access model:
- Benefits: server has full control, easier to ensure consistency
- Downsides: every operation incurs network latency, server overload

Remote File Service A Compromise

A compromise between the two extremes is necessary, allowing clients to benefit from local caching and prefetching.
This approach leads to lower latencies, reduced load on the server, and increased scalability.

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Description

Explore the basics of distributed file systems, focusing on design decisions and specifics of NFS. Learn about the research paper 'Caching in the Sprite Network File System' and understand the rationale behind design choices in the Sprite file system.

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