HA LVS Coda

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HA LVS Coda

• - Devjani Sinha

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High Availability-HA

• critical commercial applications move on the
  Internet
• An elegant highly available system may have a
  reliable group communication sub-system,
  membership management, concurrent control
  sub-system and so on.

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HA of LVS
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Coda Solution

• Coda is an advanced networked file system.
• It has been developed at CMU since 1987 by the
  systems group of M. Satyanarayanan. in the SCS
  department.
• Coda makes a distinction between servers, which
  are relatively few in number, and clients , which
  are far more numerous

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Features of Coda

• Coda has several features not found elsewhere.
  1. disconnected operation for mobile computing
  2. is freely available under a liberal license
  3. high performance through client side
  persistent caching 4. server replication and
  security model for authentication,encryption and
  access control 5. continued operation during
  partial network failures in server network 6.
  network bandwith adaptation 7. good scalability
  8. well defined semantics of sharing, even in
  the presence of network failures

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Distributed file systems

• A distributed file system stores files on one or
  more computers called servers, and makes them
  accessible to other computers called clients,
  where they appear as normal files
• The files are more widely available since many
  computers can access the servers, and sharing the
  files from a single location is easier than
  distributing copies of files to individual
  clients.
• Backups and safety of the information are easier
  to arrange since only the servers need to be
  backed up

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Coda on a client.

• typing mount will show that a file system --of
  type Coda'' -- under /coda
• A client connects to Coda'' and not to
  individual servers
• cat /coda/tmp/devjani'' to display the contents
  of a Coda file
• Coda cache manager, called Venus

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Client/Venus/Vice
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Coda with LVS

• Coda read-write operations take place at the same
  speed as those to local files
• Directory files (remember a directory is just a
  file) as well as all the attributes (ownership,
  permissions and size) are all cached by Venus,
  and Venus allows operations to proceed without
  contacting the server if the files are present in
  the cache.
• If file has been modified and it is closed, then
  Venus updates the servers by sending the new
  file.

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Volumes, Servers and Server Replication

• Files on Coda servers are not stored in
  traditional file systems
• Each volume has a directory structure like a file
  system
• Coda holds volume and directory information,
  access control lists and file attribute
  information in raw partitions
• A single volume enjoys the privilege of being the
  root volume, that is the volume which is mounted
  on /coda at startup time.
• For example, to mount the volume u.cs526 under on
  /coda/usr/cs526 the command cfs makemount u.cs526
  /coda/usr/cs526 is issued

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Disconnected Operation

• If all of the servers that an object resides on
  become inaccessible, then the client will use the
  cached copy of the object (if present) as a valid
  replica.
• Coda allows you to mark or hoard files with
  caching priorities to help keep the ones you want
  in the cache.
• Coda adapts easily to low bandwidth connections
  like (PPP or SLIP modem links).
• Reintegrate- tail -f /usr/coda/etc/console

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• Concept of hoarding files
• --Updating the hoarded files is called a hoard
  walk
• Resolution and Repair.

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Conclusion

• server replication and disconnected operation
• Unix applications without recompilation or
  relinking
• degradation due to replication is relatively
  small
• significant improvements are been made as Coda is
  refined