buffer 與cache :
通過 $ man free
buffers memory used by kernel buffers (buffers in/proc/meminfo)
cache memory used by the page cache and slabs (cached and sreclaimable in /proc/meminfo)
buff/cache sum of buffers and cache
可以看到，buffers是核心緩衝區用到的記憶體；cache是核心頁快取和slab用到的記憶體，cache大小等於cached +sreclaimable。
$man proc
buffers %lu
relatively temporary storage for raw disk blocks that shouldn't get tremendously large (20mb or so).
cached %lu
in-memory cache for files read from the disk (the page cache). doesn't include swapcached.
...sreclaimable %lu (since linux 2.6.19)
part of slab, that might be reclaimed, such as caches.
sunreclaim %lu (since linux 2.6.19)
part of slab, that cannot be reclaimed on memory pressure.
上面這段可以簡單理解為：
cpu(寫) ------> buffer ------> disk（解決cpu與硬碟效能差異，待寫資料放入buffer池）
cpu（讀）<------ cache <------ disk（命中快取，不用每次去disk獲取資料，命中率越高，io讀（bi）越小）
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 lateruse.
$ free -m
total used free sharedbufferscached
mem: 2006 1735 270 139728
-/+ buffers/cache: 967 1038
swap: 0 0 0
# total = used + free + shared
'-buffers/cache used' = 第一行used - buffers - cached - shared;
'+buffers/cache free' = 第一行free + buffers + cached + shared;
#記憶體不足，會觸發快取**，swap機制和oom。swap 越高，說明核心置換cache越頻繁，記憶體資源越緊張。
$ vmstat 1 3 #檢視bi，bi:磁碟讀，bo：磁碟寫
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
r b swpd free buff cache si sobibo in cs us sy id wa st
0 0 0 277136 40616 745644 0 088237 1188 526 7 2 91 0 0
0 0 0 277136 40632 745636 0 0 0 64 1264 628 0 0 100 0 0
0 0 0 277136 40632 745644 0 0 0 4 1258 607 0 0 100 0 0
#cache越大，bi越小
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