Clear RAM Memory Cache, Buffer and Swap Space

When you write data, it doesn’t necessarily get written to disk right then. The kernel maintains caches of many things, and disk data is something where a lot of work is done to keep everything fast and efficient.

That’s great for performance, but sometimes you want to know that data really has gotten to the disk drive. This could be because you want to test the performance of the drive, but could also be when you suspect a drive is malfunctioning: if you just write and read back, you’ll be reading from cache, not from actual disk platters.

Obviously the first thing you need to do is get the data in the cache sent on its way to the disk. That’s “sync”, which tells the kernel that you want the data written. But that doesn’t mean that a subsequent read comes from disk: if the requested data is still in cache, that’s where it will be fetched from. It also doesn’t necessarily mean that the kernel actually has sent the data along to the disk controller: a “sync” command is a request, not a command that says “stop everything else you are doing and write your whole buffer cache to disk right now!”. No, “sync” just means that the cache will be written, as and when the kernel has time to do so.

Modern Linux kernels make this a bit easier: in /proc/sys/vm/ you’ll find “drop_caches”.

You can simply echo a number to that to free caches.

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 mentioned in kernel documentation, writing to drop_cache will clean cache without killing any application/service, command echo is doing the job of writing to file.