MongoDB Version: 4.2.1

We have a sharded collection where the shard key is

and since it has random bytes it should ideally not lead to hot-spotting.

The default chunk size has not been changed (default 64MB for the version).

We are seeing that the chunks are not evenly distributed among the shards and the majority of data is residing in one shard.

Shard shard7 at shard7/mongo-7:27107,mongo-8:27107,mongo-9:27107
data : 12.78GiB docs : 3085345 chunks : 408
estimated data per chunk : 32.08MiB
estimated docs per chunk : 7562

Shard shard1 at shard1/mongo-1:27101,mongo-2:27101,mongo-3:27101
data : 12.87GiB docs : 3100445 chunks : 408
estimated data per chunk : 32.3MiB
estimated docs per chunk : 7599

Shard shard6 at shard6/mongo-6:27106,mongo-7:27106,mongo-8:27106
data : 12.78GiB docs : 3085358 chunks : 408
estimated data per chunk : 32.09MiB
estimated docs per chunk : 7562

Shard shard5 at shard5/mongo-5:27105,mongo-6:27105,mongo-7:27105
data : 12.75GiB docs : 3077756 chunks : 407
estimated data per chunk : 32.09MiB
estimated docs per chunk : 7562

Shard shard9 at shard9/mongo-1:27109,mongo-2:27109,mongo-9:27109
data : 12.75GiB docs : 3077789 chunks : 408
estimated data per chunk : 32MiB
estimated docs per chunk : 7543

Shard shard8 at shard8/mongo-1:27108,mongo-8:27108,mongo-9:27108
data : 12.79GiB docs : 3085283 chunks : 407
estimated data per chunk : 32.18MiB
estimated docs per chunk : 7580

Shard shard2 at shard2/mongo-2:27102,mongo-3:27102,mongo-4:27102
data : 104.98GiB docs : 25320636 chunks : 408
estimated data per chunk : 263.48MiB
estimated docs per chunk : 62060

Shard shard3 at shard3/mongo-3:27103,mongo-4:27103,mongo-5:27103
data : 12.76GiB docs : 3085332 chunks : 408
estimated data per chunk : 32.04MiB
estimated docs per chunk : 7562

Shard shard4 at shard4/mongo-4:27104,mongo-5:27104,mongo-6:27104
data : 12.75GiB docs : 3077839 chunks : 407
estimated data per chunk : 32.09MiB
estimated docs per chunk : 7562

Totals
data : 207.25GiB docs : 49995783 chunks : 3669
Shard shard7 contains 6.16% data, 6.17% docs in cluster, avg obj size on shard : 4KiB
Shard shard1 contains 6.21% data, 6.2% docs in cluster, avg obj size on shard : 4KiB
Shard shard6 contains 6.16% data, 6.17% docs in cluster, avg obj size on shard : 4KiB
Shard shard5 contains 6.15% data, 6.15% docs in cluster, avg obj size on shard : 4KiB
Shard shard9 contains 6.15% data, 6.15% docs in cluster, avg obj size on shard : 4KiB
Shard shard8 contains 6.17% data, 6.17% docs in cluster, avg obj size on shard : 4KiB
Shard shard2 contains 50.65% data, 50.64% docs in cluster, avg obj size on shard : 4KiB
Shard shard3 contains 6.16% data, 6.17% docs in cluster, avg obj size on shard : 4KiB
Shard shard4 contains 6.15% data, 6.15% docs in cluster, avg obj size on shard : 4KiB

The result from sh.status(true) can be found here. None of the chunks are marked as Jumbo.