Chunk size many times bigger than configure chunksize (128 MB)

[Javi_Martin (Javi Martín)]

Hi,

I just configured a new MongoDB shard with the latest version available (6.0.4). All seems to be working as expected, but when I run the “getShardDistribution” command, the estimated data per chunk is very high, much more than the default chunksize of 128 MB.

Shard zn-stg-dn-sh0 at ...
{
  data: '23.21GiB',
  docs: 3092671,
  chunks: 10,
  'estimated data per chunk': '2.32GiB',
  'estimated docs per chunk': 309267
}
---
Shard zn-stg-dn-sh1 at ...
{
  data: '23.73GiB',
  docs: 3363251,
  chunks: 4,
  'estimated data per chunk': '5.93GiB',
  'estimated docs per chunk': 840812
}
---
Totals
{
  data: '46.95GiB',
  docs: 6455922,
  chunks: 14,
  'Shard zn-stg-dn-sh0': [
    '49.44 % data',
    '47.9 % docs in cluster',
    '7KiB avg obj size on shard'
  ],
  'Shard zn-stg-dn-sh1': [
    '50.55 % data',
    '52.09 % docs in cluster',
    '7KiB avg obj size on shard'
  ]
}

As seen in the docs, “Starting in MongoDB 6.0.3, automatic chunk splitting is not performed.”. This means that the aforementioned chunk sizes seen in my ensemble are normal?

May I do something with this or is it something that we have to worry about?

Thanks in advance!

[Garaudy_Etienne]

Hey Javi,

Starting in 6.0.3, we balance by data size instead of the number of chunks. So the 128MB is now only the size of data we migrate at-a-time. So large data size per chunk is good now, as long as the data size per shard is even for the collection.

Garaudy (Sharding Product Manager)

[Javi_Martin (Javi Martín)]

Hi Garaudy,

thanks for your answer. I was worried about having large chunks (about 8 GB) but data is well balanced, so it seems to be working as expected.

Totals
{
  data: '121.08GiB',
  docs: 15634531,
  chunks: 27,
  'Shard zn-stg-dn-sh1': [
    '49.88 % data',
    '51.2 % docs in cluster',
    '7KiB avg obj size on shard'
  ],
  'Shard zn-stg-dn-sh0': [
    '50.11 % data',
    '48.79 % docs in cluster',
    '8KiB avg obj size on shard'
  ]
}

Closing this thread!
Thanks again.