cassandra 的sstables 一直增加 有2～3万多，磁盘空间也消耗的很快，经过各种修改配置调试，有点效果。
把 cassandra.yaml配置文件里的 compaction_throughput_mb_per_sec 调高点，sstables 就不会增加的那么快了。
这个配置项是控制 每秒压缩量的，压缩量大了，sstables 就不会堆积的那么快了
compaction_throughput_mb_per_sec 默认是16，我设置为 160，如下：
# Throttles compaction to the given total throughput across the entire
# system. The faster you insert data, the faster you need to compact in
# order to keep the sstable count down, but in general, setting this to
# 16 to 32 times the rate you are inserting data is more than sufficient.
# Setting this to 0 disables throttling. Note that this account for all types
# of compaction, including validation compaction.
compaction_throughput_mb_per_sec: 160