Distributed File System

1
????? ??' 5

• Distributed File System
• NFS
• AFS
• Coda

2
Sun Network File System (NFS)
3
NFS server caching

• Very similar to UNIX
• NFS version 3 offers 2 options
• Write-through caching (old one) any write
  operation is written directly to disk before
  reply is sent to the client. Why?
• Write on commit write operations will be written
  to disk when the file is closed. What is the
  advantage over the previous option?
• Write-through is required in case of partial
  failure.

4
NFS client caching

• Continuous pooling of server (timestamp based)
• Validation check
• On file open
• Every block request from server
• Check timestamp of remote file every t seconds. t
  is set adaptively
• 3-30 for files
• 30-60 for directories
• Write modified pages flushed on close or sync.
• Maximal difference between 2 copies
• 30t (sec)

5
NFS summary

• Stateless server
• Coherency achieved by constant pooling of server
• Transparency
• Access full
• Location, mobility no
• Scalability good (performance can be limited by
  hot spot files)
• File replication no (read-only replication
  only)
• Heterogeneity many implementations
• Fault tolerance good (stateless server)
• Consistency close approximation to one-copy
  sematics

6
NFS summary

• Each read/write contains full information about
  the file
• Advantages
• Fault tolerance server recovery
• Scalability no tables is kept on server
• Disadvantages
• Information on opened files does not exist
  clients should check server for updates

7
Andrew File System (AFS)
8
AFS cache consistency

• On open a copy of file and callback promise.
• On modification (server) modified file will be
  sent back to server on close and server will
  notify all clients that have callback on that
  file.
• On close if modification occurred, modified copy
  is sent back to server.
• Dramatic reduction in the number of client-server
  interactions relatively to NFS.

9
AFS summary

• Server with state
• Advantages
• Coherency well-defined approximation to one-copy
  semantics
• Minimal kernel modifications
• Dramatic reduce in server load (40 - AFS, 100 -
  NFS)
• Disadvantages
• Table of open files kept on server (can create
  problem on server failure).

10
Coda COnstant Data Availability

• Extension of AFS addressing
• Limit of scalability (no replication)
• Availability of service on server failure
• Support for mobile use of portable computers.
• Optimistic replication strategy
• For every file attached CVV (code version vector)
  table with estimated number of modifications and
  ID of the last modification for every server in
  VSG (volume storage group all the servers that
  have a replica of file)
• Modifications is sent to all currently available
  servers in VSG (AVSG)
• Callback promise from one of the servers in AVSG
  (preferred server)

11
Coda

• If CVV1(i) gt CVV2(i) for all i no conflict,
  else conflict.
• Changes propagation is on clients only
• Compare CVV for all servers in AVSG on open, send
  message in case that servers use different
  versions and set PS to latest version
• Constant check of AVSG (every T seconds (around
  10 minutes)) by client, if new servers found.
  Check volume CVV and decide on action (conflict,
  update).

12
Examples
13
Question 1

• ????? ?? ??? NFS ???? write-through cache
  (????? ???? unix ????, ????? ???). ???? ? (????
  ??? ??????)
• ?????
• ???? ??????? ??? ????? ????? ????.

14
Question 2

• ???? ?????? ?? ?????? ?caching ?? AFS ????
  scalable ???? ?? ?? NFS ?
• ?????
• Whole-file serving The entire contents of
  directories and files are transmitted to client
  computers by AFS servers
• Whole-file caching Once the copy transferred, it
  is stored in local cache, which can store
  hundreds of most frequently/recently used files.

15
Question 3

• ??? ??? ?cache ?? ?????? AFS ????? ???
  valid/invalid . ??? ??? ?? ????? ?????? ????? ??
  ??????, ?? ??? ?? ???? ?????? ??????????? ???????
  ???? ? ???!
• ?????
• ??????? ???? ?????? ????? ?? AFS. ??????, ????
  ????? ????? ???? ????? ?? token ?????? ??-token
  ?????? (valid) ????? ???? ??? ????? ??? ????.

16
Question 4

• ?? ?????? ?????? optimistic replication strategy
  ??? ????? ? Coda ?
• ?????
• Optimistic Replication Strategy ?????? ????????
  ?????, ???? ??? ???? ??? ?????? (???? ????? ???
  ?????).
• Optimistic replication strategy ????? ?? ????
  ??????? ????? ??"? ?? ???? ?????? ?????? ?? ????
  ???? ???? ?? ??????.