I used Ubuntu 16.04TLS, MongoDB 4.0.9

When I create each collection:

snappy+false
mongoshell>> db.createCollection( "test",{storageEngine:{wiredTiger:

{configString:'block_compressor=snappy, prefix_compression=false'}

}})

 

zlib+false
>mongoshell>> db.createCollection( "test",{storageEngine:{wiredTiger:

{configString:'block_compressor=zlib, prefix_compression=false'}

}})

 

I insert similar 100KB data.

total = 100KB * 100,000 = about 9GB

 

                        db.test.storageSize() 

snappy+false :    10653405184        

zlib+false :           6690177024    

 

'zlib' is compressed about 37% but 'snappy' is same the 'none'.

It was not compressed.

prefix_compress was not compressed either.

Maybe the problem is this but I do not know why.

snappy+false & snappy+true

"compression" : {
"compressed pages read" : 0,
"compressed pages written" : 0,
"page written failed to compress" : 100007,
"page written was too small to compress" : 1025
}