Remote Files

1
Remote Files
2
Traditional Memory Interfaces
Process
Secondary Memory Interface
Primary Memory Interface
Virtual Memory
File Management
Device Interface
Physical Memory
Storage Devices
3
Multics Segmented Memory
Process
Secondary Memory Interface
Primary Memory Interface
Segmented Virtual Memory
File Management
Device Interface
Physical Memory
Storage Devices
Normal data flow
Alternative data flow
4
Explicit File Copying

• Need a way for a process on one machine to pass
  info to a process on another machine
• Technique
• Sender writes a file
• User manually copies file to a remote machine
• Receiver opens the file and reads it
• Very coarse grained
• Very high latency

5
Remote Secondary Memory
Process
Secondary Memory Interface
File Management
Device Interface
Storage Devices
6
Refined View
Client Machine

• Clients server jointly implement file manager
• Disk storage is at a shared server
• Copy file contents to clients as needed

Process
Secondary Memory Interface
File Management
Server Machine
Remote Access
Remote Secondary Storage
Storage Devices
Comm Network
Storage Devices
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Remote Disk Server
file mgr diskRequest(details) VDD Pack
parameters VDD Send request (wait for
response)
(waiting for a request) RDA Unpack
parameters RDA Generate local disk
request (waiting) RDA Generate reply RDA Send
reply (waiting for a request)
VDD Receive reply VDD Unpack
parameters VDD Return to file mgr
Client Machine
General File Management
Server Machine
Virtual Disk Driver
Remote Disk Application
Storage Devices
Comm Network
Storage Devices
8
Exploiting Remote Disks

• Diskless UNIX workstation
• X Terminal
• Network computer (thin client)
• Palmtops and PDAs

9
Performance Reliability

• Became commercially feasible in about 1986
• Biggest concern was reliability
• Use datagrams for performance
• How can the system assure that commands get
  executed?
• If the server crashes
• How can we prevent incomplete transactions?
• How can the client resume its operation?

10
Reliable Command Execution

• Using request time-outs
• Issue command
• Wait for a fixed ?t
• Re-issue the command
• Failed command send operation
• Re-issue is appropriate
• Failed command ACK operation
• Is re-issued command appropriate?
• Idempotent operations

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Crash Recovery

• Distributed state
• Clients view of open files
• Servers view of open files
• Distribution ? less network traffic
• Distribution ? recovering state is difficult
• Server must have stable memory
• Can be built without special hardware, but
  expensive
• Stateless servers
• All state is kept at the client
• All operations are idempotent
• Stateless server ? more network traffic
• Stateless server ? no server-side state recovery

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Remote File Server

• Read/write management
• Pack/unpack byte stream
• Buffering
• ...

• Block management
• Buffering
• Device management
• ...

Client Machine
General File Management
Server Machine
Client Part of Remote File System
Server Part of Remote File System
Storage Devices
Comm Network
Storage Devices
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Block Caching

• Widely used in all file systems
• In RFS can buffer at
• Server
• Doesnt avoid network latency
• Client
• Consistency
• Sometimes use sequential write consistency (no
  sharing if there are multiple writers)

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Crash Recovery

• Client has a file open and server crashes
• Distributed state makes recovery difficult
• Can counteract with a stateless server
• But it requires that state be transmitted with
  every service request
• Recovery-oriented file service, e.g., Sun NFS
• Performance-oriented file service

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File-Level Caching

• Copy an entire file to the client when it is
  needed
• Requires enough client storage to hold any file
• Widely-used in contemporary palmtops
• File Consistency
• Synchronization on any file write operation
• Immutable files

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Directories -- Pathnames

• Need a path through the internet to host in
  addition to path from root at the host
• goober/usr/bernie/good/stuff
• /../goober/usr/bernie/good/stuff
• Remote mounting
• Mount a file system on a remote host (use a
  superpathname to reference server)
• Use normal path names

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Opening a File
Machine R
/
Machine S

usr
bin
etc
/
zip
mt_pt

usr
bin
etc
mount
Machine T
s_mnt
/
s_hop
mount

usr
bin
etc
zip
t_mnt
t_file