Storage Smarts Answer #39: Synchronous replication

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QUESTION OF THE WEEK:

This is one method of ensuring data availability in which every write written to the primary side is written "in-order" to the remote side before the application sees an "I/O complete" message.

a. timed backup
b. local backup
c. synchronous replication
d. asynchronous replication
e. duplication

Were you correct? This week's answer is:
c. synchronous replication

According to storage networking expert Christopher Poelker, under synchronous replication an I/O is not complete until it is written to both sides. This is a good thing because your transactions stay consistent. Every write written to the primary side is written "in-order" to the remote side before the application sees an I/O complete" message. The problem here is that Fibre Channel protocol requires four round trips to complete every I/O under sync replication. Even using dark Fibre cables between sites the speed of light becomes your limiting factor because of the four round trips, you lose about a millisecond for every 25 miles. Sync is limited in distance to about 100 kilometers. After that, application performance goes in the toilet. Async can go around the planet. So the farther you go, the more you need async remote copy.

For more information:

Seven steps to data replication

Synchronous vs. asynchronous

Data replication for disaster recovery

Looking at two-way replication methods

Ask W. Curtis Preston your backup and recovery questions.

Do you have an idea for a quiz question topic? Let us know!


This was first published in January 2003

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