Storage apps keep Exchange running 24/7


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Lotus/Domino availability

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Lotus/Domino, the other major enterprise e-mail application, minimizes e-mail availability problems by:
  • Being architected for frequent replication
  • Continuously managing and maintaining replication
  • Deploying multiple small servers, each handling relatively few users
Source: David Yavin, co-founder and chief strategy officer, DYS Analytics Inc.

For example, the continuous allocation and release of various memory blocks within an Exchange process leads to virtual memory fragmentation. According to Microsoft, when virtual memory is available for a process but the virtual memory blocks aren't large enough, fragmentation occurs over time. Virtual memory issues are more prevalent in a clustered Exchange (2000 and 2003) configuration because these environments are typically used to scale Exchange to host thousands of users with multiple storage groups and messaging databases. In short, Exchange clusters aren't for the inexperienced.

Standard e-mail backup doesn't cut it. "There's a lot of data and it's changing rapidly," says Lee Benjamin, an analyst at Ferris Research in San Francisco. Just do the math: A company with 240 mailboxes and a 250MB quota will end up with a 60GB Exchange data store. Backing it up will take 3.5 hours; restoring it will take another 3.5 hours, calculates Keith McCall, chief technology officer at Azaleos Corp., a high-availability Exchange appliance provider in Redmond, WA.

"How many users will wait 3.5 hours for e-mail access?" he asks.

Further complicating matters is the way Exchange stores data. Referred to as the information store, there are two separate databases: one manages data in user mailboxes, while the other handles public folders. In addition, the two databases must work with the Exchange database engine, which handles disk storage and memory, and also manages the caching between disk and memory. IT managers complain in online postings that the Exchange information store is prone to corruption. Microsoft postings describe the many conditions under which the Exchange database may become corrupted, such as during virus scanning. Lotus/Domino is architected differently than Exchange and doesn't present the same problems (see "Lotus/Domino availability," this page).

High-availability solutions must take pains not to replicate any database corruption. "One of the things we liked about Teneros [an e-mail appliance from Teneros Inc., Mountain View, CA] is that it replicates object by object," says William Cumming, director of information technology at Strome Investment Management, a small investment firm in Santa Monica, CA. "That avoids corruption." Other approaches operate at the bit level, which can replicate corrupted data, although bit-level recovery can be much faster.

This was first published in September 2006

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