昨天晚上凌晨12点接到监控短信（dataguard is down），于是登录系统查看原因，首先查看备库的alertlog文件，查看最近的半小时的l
/proc/sys/vm/drop_caches (since linux 2.6.16)
writing to this file causes the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free.
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
as this is a non-destructive operation, and dirty objects are not freeable, the user should run "sync" first in order to make sure all cached objects are freed.
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