Major Description
The default value of hbase.regionserver.global.memstore.lowerLimit of 25% is very wrong and in almost all cases was problematic (I've seen this in at least 3 occurrences). The cost of flushing a memstore is fairly high and when the global size reaches 40% then ALL inserts are blocked. This means that with a heap of 1GB you could be flushing for 10-20 seconds or worse.
I suggest a default setting of 38% or even 40% so that only a region or two will be flushed (the biggest ones) for maximum availability.