Disconnected Operation in the Coda File System

Notes by Gabriel Weisz ( glw3@cornell.edu )

Goals

• Coda is a distributed file system meant to supercede AFS
• Allow transparent operation (if possible) when clients are disconnected from servers
• Allow transparent reintegration for reconnecting clients if possible

Strategies

• The client's local hard drive acts as a cache for files on servers
• Files are then accessed through normal system calls
• This preserves transparency
• Requests for files not on the client when disconnected result in failures at the system call level
• Coda uses an optimistic strategy for replica control
• Automatically caches some files, but allows users to specify files to be cached

Implementation Details

The client has a small in kernel cache and a user level process (Venus) to actually manage the cache

Three states for Venus (hoarding, emulation, and reintegration)

User editable hoard profiles to prioritize and specify files

Hoard walking to refresh files

A cache coherence protocol is used to attempt to preserve consistency for in cache data

A callback message is sent from servers to clients when data becomes invalid

All transactions are logged on the client side when disconnected, and then replayed upon reconnection

Users can request cache updates if they know that disconnection is immenent

Disconnected clients creating new files first go through a small number of preallocated permanent file ids, and later new files are given temporary file ids until reintegration

There's a notion of servers containing class one replicas while clients contain class 2 replicas

Questions

How do the 2 replica classes really come into play?

Are conflicts really as rare as they claim (it would seem that, especially on large programming projects, conflicts could easily occur when editing header files and such)

What happens if there's a system crash during reintegration?

How fast is it to fetch files during connected operation (and during reintegration)?

What happens if a callback message is lost?

Can the system be extended to work with other devices such as palmtops?