Critical - P2 ISSUE SUMMARY AND IMPACT
Capped collection handling in WiredTiger is inefficient because of way that WiredTiger tracks and expires documents in the capped collection.
WiredTiger uses a specific internal cursor to find the "beginning of the capped collection". Combined with asynchronous deletion of expired capped collection records, this is inefficient for collections with high numbers of inserts because requests have to process large number of expired documents.
USER IMPACT
Capped collection performance degrades over time. Note that the oplog is a capped collection, so users running replica sets with WiredTiger may be impacted by this issue even if no other capped collections are used.
RESOLUTION DETAILS
WiredTiger now caches the current "first" unexpired document in a capped collection. This change improves performance for all capped collections, but is particular important for the performance of replication because the oplog depends on capped collection performance.
AFFECTED VERSIONS
MongoDB 3.0.0 through 3.0.4.
FIX VERSION
The fix is included in the 3.0.5 production release.
Original description
Over the course of 2-3 days, running a simple insert workload with hammer, the performance of inserts degrades from about 20K/s to about 4K/s.
This was a single node replica set, writing locally to the master, using --nojournal to avoid intertwining any other potential issues.
Perf indicates this:
- 99.99% __curfile_next mongo::WiredTigerRecordStore::cappedDeleteAsNeeded_inlock(mongo::OperationContext*, mongo::RecordId const&)
- is depended on by
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WT-1973 MongoDB changes for WiredTiger 2.7.0
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- Closed
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- is duplicated by
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- Closed
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- Closed
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- is related to
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- Closed
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- Closed
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- related to
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SERVER-19178 In WiredTiger capped collection truncates, avoid walking lists of deleted items
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- Closed
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