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As operations modify the table more and more delta files are created and need to be compacted to maintain adequate performance.  There are two three types of compactions, minor, major and majorrebalance.

• Minor compaction takes a set of existing delta files and rewrites them to a single delta file per bucket.
• Major compaction takes one or more delta files and the base file for the bucket and rewrites them into a new base file per bucket.  Major compaction is more expensive but is more effective.
• More information about rebalance compaction can be found here: Rebalance compaction

All compactions are done in the background. Minor and major compactions do not prevent concurrent reads and writes of the data.  . Rebalance compaction uses exclusive write lock, therefore it prevents concurrent writes. After a compaction the system waits until all readers of the old files have finished and then removes the old files.

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