In Linux, data written to the filesystem is maintained in pagecache until it must be written out to disk (due to an application-level fsync or the OS's own flush policy). The flushing of data is done by a set of background threads called pdflush (or in post 2.6.32 kernels "flusher threads").
在Linux中，写入文件系统的数据保存在页缓存，知道它必须被写入到磁盘（由应用程序级fsync或系统自己的冲洗策略）。数据的冲洗是通过一组后台线程调用pdflush（或在post 2.6.32内核 ”冲洗器线程”）完成的。
Pdflush has a configurable policy that controls how much dirty data can be maintained in cache and for how long before it must be written back to disk. This policy is described here. When Pdflush cannot keep up with the rate of data being written it will eventually cause the writing process to block incurring latency in the writes to slow down the accumulation of data.
You can see the current state of OS memory usage by doing
> cat /proc/meminfo
The meaning of these values are described in the link above.
Using pagecache has several advantages over an in-process cache for storing data that will be written out to disk:
The I/O scheduler will batch together consecutive small writes into bigger physical writes which improves throughput.
The I/O scheduler will attempt to re-sequence writes to minimize movement of the disk head which improves throughput.
It automatically uses all the free memory on the machine