In certain queries, where there isn't an index available, wiredTiger takes way too long, when comparison with MMAPv1.
In the example below the difference is 6min vs 0.2 sec.
Example:
db.forms.find({'dimension': 2,'degree':2 }).count()
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And, there isn't any entry with the key "degree".
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wiredTiger:
config:
engine: 'wiredTiger'
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wiredTiger:
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collectionConfig:
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blockCompressor: none
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We also have tried with snappy and zlib.
The filesystem is ext4, and we have also tried xfs without a significant difference (when compared to MMAPv1).
log entry:
2016-06-28T05:09:22.454+0000 I COMMAND [conn4] command hmfs.forms command: count { count: "forms", query: { dimension: 2, degree: 2 } } planSummary: IXSCAN { dimension: -1, field_label: 1, level_norm: -1 } fromMultiPlanner:1 keyUpdates:0 writeConflicts:0 numYields:13831 reslen:62 locks:{ Global: { acquireCount: { r: 27664 } }, Database: { acquireCount: { r: 13832 } }, Collection: { acquireCount: { r: 13832 } } } protocol:op_query 369464ms
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MMAPv1
config:
log entry:
2016-06-28T05:10:12.412+0000 I COMMAND [conn15] command hmfs.forms command: count { count: "forms", query: { dimension: 2, degree: 2 } } planSummary: IXSCAN { dimension: -1, field_label: 1, level_norm: -1 } keyUpdates:0 writeConflicts:0 numYields:447 reslen:62 locks:{ Global: { acquireCount: { r: 896 } }, MMAPV1Journal: { acquireCount: { r: 448 } }, Database: { acquireCount: { r: 448 } }, Collection: { acquireCount: { R: 448 } } } protocol:op_query 181ms
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This two examples were run on the same machine, on different empty dbs where the only difference is the storage engine. In both cases, the collections was restored from the same dump file.
Perhaps we are doing something wrong, if so, please enlighten us, as we are planning switch all our servers to MMAPv1.
Bug
Major - P3
WT-2670 Inefficient I/O when read full DB (poor readahead)