linux buff清理

• 查看使用情况
• sync命令将文件系统buffer里面的数据flush到磁盘上

To free pagecache:  echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes:  echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:  echo 3 > /proc/sys/vm/drop_caches