Memory type                                Abbr
Physical memory available for HBase RS     Total
Memory size for memstore                   MSize
Memory for block cache (L1) cache          L1Size
Memory for JVM related components          JHSize

-Xmx                        Xmx = MSize + L1Size + JHSize
-XX:MaxDirectMemorySize     DMen = Total - Xmx

hbase.regionserver.global.memstore.size              ULimit = MSize / Xmx
hfile.block.cache.size                               blksz = 0.8 - ULimit
hbase.bucketcache.size                               bucsz = DMen + (blksz * Xmx)
hbase.bucketcache.percentage.in.combinedcache        ccsz = 1 - ((blksz * Xmx) / bucsz)
hbase.bucketcache.ioengine                           ioeng = offheap or file:/localfile

Total      20480
MSize      1024 
L1Size     1024
JHSize     512
Xmx        2560       
DMen       17920
ULimit     0.4
blksz      0.4
bucsz      18944
ccsz       0.946
ioeng      offheap