Disabling disk cache in linux

Question

In a class project my teacher told us to make some code evaluations (C language) and to do so we need to disable the disk caching during the tests.

Currently I'm using Ubuntu 12.04, how can I do this?

Thanks.

Answer 1

You need root access to do this. You can run hdparm -W 0 /dev/sda command to disable write caching, where you have to replace /dev/sda with device for your drive:

#include <stdlib.h>
...
system("hdparm -W 0 /dev/sda1");

You can also selectively disable write caching to individual partitions like this: hdparm -W 0 /dev/sda1.

To reenable caching again just use the -W 1 argument.

man hdparm, man system

Answer 2

I think you need to tell your teacher that it's no longer 1984. Modern computer systems have dozens of caches and there is no good way to disable them all:

• Cache on the hard disk itself
• Caches in the I/O hardware subsystem
• Caches in the virtual file system
• Several levels of caches in the CPU

So the question is what you want to test and which caches you want to disable for this.

Answer 3

echo 100 > /proc/sys/vm/dirty_expire_centisecs

echo 100 > /proc/sys/vm/dirty_writeback_centisecs

this reduce to 1 second the flush from the RAM to disk

you can test with 0

or :

echo 1 > /proc/sys/vm/drop_caches

to flush all RAM to disk