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Type: Bug
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Resolution: Done
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Priority: Major - P3
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None
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Affects Version/s: 2.6.10
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Component/s: Sharding
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Labels:None
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ALL
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An issue has been found where queries which were targeted to a single shard suffered significant performance degradation relative to running the same query against an unsharded collection. The issue was tracked down to an unnecessary mergecursors operation being performed on the primary shard (SERVER-7656). A patched version of 2.6 was subsequently produced to prevent this unnecessary step from being performed however there is an open question as to why the performance of the mergecursrors operation is so bad when it is essentially a noop.
In order to assist engineering with debugging this issue, I have identified a series of steps than can be followed in order to reliable reproduce this issue on MongoDB 2.6.10 (see 'Steps to reproduce').
The SERVER-7656 patch will clearly stop this from happening for queries targeted to a single shard however there is an open question as to why merging pre-sorted results from a single shard is taking as long as it is.