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[[Synchronization|Synchronous]] replication guarantees "zero data loss" by the means of [[atomic operation|atomic]] write operations, where the write operation is not considered complete until acknowledged by both the local and remote storage. Most applications wait for a write transaction to complete before proceeding with further work, hence overall performance decreases considerably. Inherently, performance drops proportionally to distance, as minimum [[Latency (engineering)|latency]] is dictated by the [[speed of light]]. For 10 km distance, the fastest possible roundtrip takes 67 μs, whereas an entire local cached write completes in about 10–20 μs.
In [[Asynchronous I/O|asynchronous]] replication, the write operation is considered complete as soon as local storage acknowledges it. Remote storage is updated with a small [[Latency (engineering)|lag]]. Performance is greatly increased, but in case of a local storage failure, the remote storage is not guaranteed to have the current copy of data (the most recent data may be lost).
Semi-synchronous replication typically considers a write operation complete when acknowledged by local storage and received or logged by the remote server. The actual remote write is performed asynchronously, resulting in better performance but remote storage will lag behind the local storage, so that there is no guarantee of durability (i.e., seamless transparency) in the case of local storage failure.{{citation needed|date=September 2009}}
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