About CERN VM File System

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What is CERN VM File System?

CERN VM File System is a client server file system developed to deliver software distributions onto virtual machines in a fast, scalable and reliable way.

Reliability and Load Balancing in CVMFS

CVMFS can use multiple mirror servers while accessing files from repositories. To configure which mirror servers to use and in which order, the CVMFS_SERVER_URL parameter in /etc/cvmfs/default.local file can be set to a semicolon separated list of mirror servers enclosed in double quotes. From the CVMFS technical report:

“When-ever download of files fails from a server, CernVM-FS automatically switches to the next mirror server. Additionally, on the first download and after every 1000 down- loads, CernVM-FS orders the list of servers according to the round trip time for downloading a file; it then switches automatically to the closest server.”

Currently, this helps handle load balancing and reliability.

Bringing DNS into the Equation

The idea is to help CVMFS clients connect to the mirror server closest to them, allowing lowest latency access to files. When a client attempts to connect to a server, the name resolution process would lead to the authoritative nameserver for CVMFS. This DNS server can be configured such that it replies with the A (address) record of the mirror server closest to the client. After reading up online, I ended up on this post. According to the answers, there are 2 possible ways to achieve this:

  • PowerDNS with geobackend

  • BIND with ACL

I now require to read about these approaches, their implementation details and weigh the pros and cons of both.