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Type:
Improvement
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Resolution: Fixed
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Priority:
Major - P3
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Affects Version/s: 5.0.0-rc1
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Component/s: Sharding
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Fully Compatible
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v5.0
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Sharding EMEA 2021-07-26, Sharding EMEA 2021-08-09
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As discovered during the investigation of SERVER-57309, the update of config.chunks entries during setFCV - happening under the chunk operations lock - may block migrations for long periods in presence of a lot of chunks.
Following, the results of some simple downgrade/upgrade experiments with different number of chunks (executed on the virtual workstation):
| downgrade | downgrade | upgrade | upgrade | |
|---|---|---|---|---|
| update operation number of chunks |
$set ns | $unset uuid & timestamp | $set uuid & timestamp | $unset ns |
| 100K | 32 s | 27 s | 22 s | 31 s |
| 500K | 180 s | 164 s | 117 s | 144 s |
| 1M | 324 s | 313 s | 211 s | 271 s |
A possible improvement to reduce the window in which balancing is not allowed would be to perform the $unset operations outside such lock, but still during setFCV.