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We have huge collections ( around 3.5 TB ) that is sharded with 2 shards on mongodb. Since the collection is growing and load is getting increased on existing shards we decided to add 2 new shards for this cluster. However we see data is filling up very slowly on new shards , while still keeping high on old shards ( after a week on one collection we have around 1.7% of data on new shards, while 48% on old once , and second collection is even less - 0.5% ). All data in both collections is having ttl expiring indexes of 2 months, however it looks like this mechanism is also not improving the process ( theoretically data should get expired in old shards and therefore the balance should become better )

Is there anything anything that can be done to speed up migration of data to new shards?

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  • What is the output of sh.status() for this collection? Maybe try sh.enableAutoSplit in case you run MongoDB version 7. Or try configureCollectionBalancing Jan 22 at 7:18
  • The output says balancing true, The number of chunks for one collection is 15K for shards 1 and 2 , and approximately 1.5K for shards 3 and 4 For second the number of chunks for shards 1 and 2 are 63-65K and 1-1.1K for shards 3 and 4 Unfortunately i am running 4.4 , so both commands are not available Jan 22 at 13:17
  • That's really a lot. As mentioned already in another comment, you should consider Hashed Sharding. MongoDB version 4.4 will go end-of-life in one week. Jan 22 at 14:22
  • We do use hashed sharding, and it was working great with almost equal balance between 2 shards. T Jan 22 at 18:39

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