Disk block size for mongodb deployment
Why does Mongodb production notes have no recommendations about the
  disk block size?


As per MongoDB documentation Configure Block Size in a Blockstore When you back up your deployment to a Blockstores, the Backup Daemon first takes a snapshot of the head database. It then breaks this snapshot into blocks and writes these blocks to the blockstore.

The different blockstores have different default sizes and ranges of sizes for blocks.

Blockstore Type Block Size Range    Default Size
MongoDB          64 KB to 15 MB          64 KB
S3               64 KB to 16 MB          16 MB


In general, increasing block size results in faster snapshots and restores, but requires more disk space. These competing factors should be considered when determining if you wish to tune the block size.

There are two exceptions where performance improves as the block size increases without requiring additional disk space:

Workload                      Impact
Update- and Delete-Intensive  No matter how small you make the block size, the entire block file is rewritten. Since the entire file is always rewritten, there is no difference in storage space if you change the block size.
Insert-only                   The existing blocks never change. As you increase block size, block management is simplified. This enables the best possible performance on snapshot and restore.


The range of minimum block sizes include:

64 KB
128 KB
256 KB
512 KB
1 MB
2 MB
4 MB
8 MB
15 MB (MongoDB) or 16 MB (S3)



  Does disk block size have any impact on mongodb performance in
  practice?


As per Blog from @Colt McAnlis, here  “The default block size on volumes is 4K. For throughput-oriented workloads, values of 256KB or above are recommended.”

It’s important to observe the following formula wrt disk performance:

Throughput (M/sB) = IOPS * Blocksize.


How this evens out is that IOPS and block Size tend to have an inverse relationship to each other. As block size increases, it takes longer latency to read a single block, and thus the # of IOPS decreases. Inversely, smaller block sizes yield higher IOPS.

For further your ref here