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From the Chief Technology Officer at Marathon Technologies

Jerry Melnick

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UNDERSTANDING DIALABLE AVAILABILITY


As many of you know, one of the key components of everRun VM is the ability to dial up or dial down the level of availability needed to protect business-critical applications. With buzz surrounding the release of Citrix’ XenServer 5, we have been approached with questions like “what should I use to protect my low-priority applications” and “how do I know when something should or shouldn’t be protected with the lockstep option?” To help explain the three levels of availability and when they would be used, we’ve put together these tips:

LEVEL 1: BASIC FAILOVER WITH XENSERVER HA
The first level of availability, basic failover and recovery, is appropriate for applications where recovery is not absolutely critical, and where manual intervention, while not desirable, is acceptable. These may include infrastructure applications or dev and test systems.
XenServer HA provides:

  • Basic failover to another host within the same Xen pool, with resource calculation to determine whether adequate resources are available within the pool to handle a defined number of simultaneous host failures (XenServer HA does not check the health of available devices, such as network and storage)
  • Monitoring of health of the hosts within a pool (Network and storage health are not monitored)
  • No storage or data protection – using this level requires a shared-storage configuration

LEVEL 2: COMPONENT-LEVEL FAULT TOLERANCE WITH everRun VM
For applications with business-critical roles, everRun VM provides component-level fault tolerance: the ability to withstand the loss of an individual network or storage component without interruption or downtime.
The attributes of Level-2 availability include:

  • Automated setup and fault management: policies handle system, network and disk I/O failures without IT intervention
  • Assured recovery of virtual machines
  • Zero downtime due to I/O failures and zero data loss
  • Synchronous data mirroring between hosts; no need for shared storage
  • Continuous active validation of all components on production and standby system to ensure complete redundancy at all times for recovery in the event of a failure
  • Comprehensive availability including system, network, and data availability, all in one integrated solution

LEVEL 3: SYSTEM-LEVEL FAULT TOLERANCE WITH everRun VM AND LOCKSTEP OPTION
For the most mission-critical systems, Marathon everRun VM with Lockstep Option provides system-level fault tolerance, with continuous availability in the face of component or system-wide failures. Level 3 will be available in 2009 and offers protection for systems that cannot experience any downtime and must maintain transaction state at all costs. everRun VM with Lockstep Option offers all of the benefits of everRun VM (Level 2), together with:

  • Zero downtime even for complete host failures
  • Application state maintained during failures
  • Memory state maintained during failures

For more information on the different levels of availability please visit here.

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More Stories By Jerry Melnick

Jerry Melnick (jmelnick@us.sios.com) is responsible for defining corporate strategy and operations at SIOS Technology Corp. (www.us.sios.com), maker of SIOS SAN and #SANLess cluster software (www.clustersyourway.com). He more than 25 years of experience in the enterprise and high availability software industries. He holds a Bachelor of Science degree from Beloit College with graduate work in Computer Engineering and Computer Science at Boston University.