centos清理缓存

释放网页缓存(To free pagecache):
echo 1 > /proc/sys/vm/drop_caches

释放目录项和索引(To free dentries and inodes):
echo 2 > /proc/sys/vm/drop_caches

释放网页缓存，目录项和索引（To free pagecache, dentries and inodes）:
echo 3 > /proc/sys/vm/drop_caches