Hi All,

The query is to clarify myself on the "buffers" and "cache" column from the output of free command.

This is what my understanding...

Buffer is something where data is there in memory but yet to be flushed to disk . The data will be flushed to disk by bdflush daemon periodically or we can do it manually by running sync command .

Cache on the other hand is program/data which is loaded into memory but is retained in memory so that if is needed again , it will be quickly available.

To understand the concept of buffers , I tried the following experiment...

This is the reading of free command in my desktop

[zama@localhost ~]$ free -m
total used free shared buffers cached
Mem: 2897 465 2431 0 30 230
-/+ buffers/cache: 204 2692
Swap: 4000 0 4000

[zama@localhost ~]$ sync

[zama@localhost ~]$ free -m
total used free shared buffers cached
Mem: 2897 466 2431 0 30 230
-/+ buffers/cache: 205 2691
Swap: 4000 0 4000

Here I cannot see buffer getting reduced after executing the sync command.

Next I tried the following...Tried to write a huge file to the disk .

[zama@localhost ~]$ dd if=/dev/zero of=test bs=1024k

As expected , the cached value should increase and free is confirming this..

@localhost ~]# free -m
total used free shared buffers cached
Mem: 2897 1466 1430 0 32 1127
-/+ buffers/cache: 306 2590
Swap: 4000 0 4000

I again executed the sync command and then checked using free . I can see that the buffer value getting decreased from the output of free command . There was no reduction in the cache . This means that the dirty pages in RAM after my execution of dd command has been flushed to disk .

@localhost ~]# free -m
total used free shared buffers cached
Mem: 2897 1466 1430 0 10 1127
-/+ buffers/cache: 306 2590
Swap: 4000 0 4000

Then I updated the drop_cache kernel parameter so that the cache value is dropped

[root@localhost ~]# cat /proc/sys/vm/drop_caches
0

[root@localhost ~]# echo "1" > /proc/sys/vm/drop_caches

[root@localhost ~]# cat /proc/sys/vm/drop_caches
1

I can see that cache value is dropped , but also buffer value is also dropped. free confirms this..

[root@localhost ~]# free -m
total used free shared buffers cached
Mem: 2897 299 2597 0 1 74
-/+ buffers/cache: 224 2672
Swap: 4000 0 4000

So , my initial statement that "Buffer" is RAM data which is yet to be flushed to disk looks to be correct .

Please guide me whether I am in the right direction ..

Thanks in Advance

I never really thought of buffers vs cache before, but you seem to have a solid case in your way of reasoning. I would agree to your statements.

Quoted from the #man free

I'm pretty sure your distinction between buffers and cache is completely wrong.

I don't fully understand the distinction myself, but I think the underlying issue behind the distinction is page alignment.

Caching in a paged OS is simplest when an entire page (normally 4KB) is aligned on a page boundary in the file itself, in physical memory of the cache and in the virtual address mapping that uses it (user address space if it is a mapped page or kernel address space for other forms of file I/O).

I believe cache is used for caching things that can be handled as full aligned pages and buffers are used for caching things that can't.

That should be orthogonal to the distinction between data that has been modified and is awaiting write back and data that is kept in case it will be read again.