【cassandra】max_mutation_size和commitlog_segment_size

# The size of the individual commitlog file segments.  A commitlog
segment may be archived, deleted, or recycled once all the data
in it (potentially from each columnfamily in the system) has been
flushed to sstables.
commitlog_segment_size_in_mb是單次提交的日誌文件段的大小。
當日志文件段裏全部數據（可能來自系統中各個列族）都被刷到磁盤文件sstables中後，日誌文件段就能夠被存檔，刪除或者重複利用。
#
# The default size is 32, which is almost always fine, but if you are
# archiving commitlog segments (see commitlog_archiving.properties),
# then you probably want a finer granularity of archiving; 8 or 16 MB
# is reasonable.
默認大小是32MB，一般都很合適。
可是若是你開啓了：日誌文件段歸檔功能（詳情見配置commitlog_archiving.properties）
這時候，你最好把值調成8或者16MB。
# Max mutation size is also configurable via max_mutation_size_in_kb setting in
# cassandra.yaml. The default is half the size commitlog_segment_size_in_mb * 1024.
# This should be positive and less than 2048.
另外一個最大差別大小（大於它就要寫盤了）在參數max_mutation_size_in_kb中定義，默認值
是commitlog_segment_size_in_mb大小的通常（乘1024，切換單位）
# NOTE: If max_mutation_size_in_kb is set explicitly then commitlog_segment_size_in_mb must
# be set to at least twice the size of max_mutation_size_in_kb / 1024
#
注意：若是max_mutation_size_in_kb這個值被明確設置了，那麼commitlog_segment_size_in_mb也必須被設置
而commitlog_segment_size_in_mb至少要是max_mutation_size_in_kb / 1024的兩倍
（也就是max_mutation_size是16M的話，commitlog_segment_size至少是32M）
commitlog_segment_size_in_mb: 32