In Linux, what is the difference between “buffers” and “cache” reported by the free command?

Question

This is an old question that I've seen from time to time. My understanding of it is rather limited (having read about the differences a long time ago, but the factoid(s) involved never really stuck).

As I understand it,

• Buffers

  Are used by programs with active I/O operations, i.e. data waiting to be written to disk

• Cache

  Is the result of completed I/O operations, i.e. buffers that have been flushed or data read from disk to satisfy a request.

Can I get a clear explanation for posterity?

Answer 1

The "cached" total will also include some other memory allocations, such as any tmpfs filesytems. To see this in effect try:

mount -ttmpfs none t
dd if=dev/zero of=t/zero.file bs=10240 count=10240
sync; echo 3 > /proc/sys/vm/drop_caches; free -m
umount t
sync; echo 3 > /proc/sys/vm/drop_caches; free -m

and you will see the "cache" value drop by the 100Mb that you copied to the ram-based filesystem (assuming there was enough free RAM, you might find some of it ended up in swap if the machine is already over-committed in terms of memory use). The "sync; echo 3 > /proc/sys/vm/drop_caches" before each call to free should write anything pending in all write buffers (the sync) and clear all cached/buffered disk blocks from memory so free will only be reading other allocations in the "cached" value.

The RAM used by virtual machines (such as those running under VMWare) will also be counted in free's "cached" value, as will RAM used by currently open memory-mapped files.

So it isn't as simple as "buffers counts pending file/network writes and cached counts recently read/written blocks held in RAM to save future physical reads", though for most purposes this simpler description will do.

Answer 2

Tricky Question. When you calculate free space you actually need to add up buffer and cache both. This is what I Could find

A buffer is something that has yet to be "written" to disk. A cache is something that has been "read" from the disk and stored for later use.

http://visualbasic.ittoolbox.com/documents/difference-between-buffer-and-cache-12135

Answer 3

Freeing buffer/cache

Warning This explain a strong method not recommended on production server! So you're warned, don't blame me if something goes wrong.

For understanding, the thing, you could force your system to delegate as many memory as possible to cache than drop the cached file:

Preamble

Before of doing the test, you could open another window an hit:

$ vmstat -n 1
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 0  1  39132  59740  39892 1038820    0    0     1     0    3    3  5 13 81  1
 1  0  39132  59140  40076 1038812    0    0   184     0 10566 2157 27 15 48 11
...

for following evolution of swap in real time.

Nota: You must dispose of as many disk free on current directory, you have mem+swap

The demo

$ free
         total       used       free     shared    buffers     cached
Mem:       2064396    2004320      60076          0      90740     945964
-/+ buffers/cache:     967616    1096780
Swap:      3145720      38812    3106908

$ tot=0
$ while read -a line;do
      [[ "${line%:}" =~ ^(Swap|Mem)Total$ ]] && ((tot+=2*${line[1]}))
    done </proc/meminfo
$ echo $tot
10420232

$ dd if=/dev/zero of=veryBigFile count=$tot
10420232+0 records in
10420232+0 records out
5335158784 bytes (5.3 GB) copied, 109.526 s, 48.7 MB/s

$ cat >/dev/null veryBigFile

$ free
             total       used       free     shared    buffers     cached
Mem:       2064396    2010160      54236          0      41568    1039636
-/+ buffers/cache:     928956    1135440
Swap:      3145720      39132    3106588

$ rm veryBigFile 

$ free
         total       used       free     shared    buffers     cached
Mem:       2064396    1005104    1059292          0      41840      48124
-/+ buffers/cache:     915140    1149256
Swap:      3145720      39132    3106588

Nota, the host on wich I've done this is strongly used. This will be more significant on a really quiet machine.