Merge pull request #4924 from ipfs/docs/config

Add information about bloom filter to config.md

docs/config.md

@@ -146,8 +146,13 @@ A boolean value. If set to true, all block reads from disk will be hashed and
 verified. This will cause increased CPU utilization.
 
 - `BloomFilterSize`
-A number representing the size in bytes of the blockstore's bloom filter. A
-value of zero represents the feature being disabled.
+A number representing the size in bytes of the blockstore's [bloom filter](https://en.wikipedia.org/wiki/Bloom_filter). A value of zero represents the feature being disabled.  
+
+This site generates useful graphs for various bloom filter values: <https://hur.st/bloomfilter/?n=1e6&p=0.01&m=&k=7>  
+You may use it to find a preferred optimal value, where `m` is `BloomFilterSize` in bits. Remember to convert the value `m` from bits, into bytes for use as `BloomFilterSize` in the config file.  
+For example, for 1,000,000 blocks, expecting a 1% false positive rate, you'd end up with a filter size of 9592955 bits, so for `BloomFilterSize` we'd want to use 1199120 bytes.  
+As of writing, [7 hash functions](https://github.com/ipfs/go-ipfs-blockstore/blob/547442836ade055cc114b562a3cc193d4e57c884/caching.go#L22) are used, so the constant `k` is 7 in the formula.
+
 
 Default: `0`