Conclusion
- Buffer refers to cache for block devices.
- Cache refers to cache for the filesystem.
These caches include both read and write operations.
Experimental Proof
First, check the values of buffer and cache:
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| root@localhost:~# cat /proc/meminfo | grep -E 'Buffers|Cache'
Buffers: 238024 kB
Cached: 1041304 kB
SwapCached: 88328 kB
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Then use dd to read 100MB from a block device. As shown in the result, the buffer increased by 100MB.
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| root@localhost:~# dd if=/dev/vda of=/dev/null bs=1M count=100
记录了100+0 的读入
记录了100+0 的写出
104857600字节（105 MB，100 MiB）已复制，0.334603 s，313 MB/s
root@localhost:~# cat /proc/meminfo |grep -E 'Buffers|Cache'
Buffers: 341480 kB
Cached: 1042240 kB
SwapCached: 88328 kB
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Reading the same block device again shows a significantly higher speed, indicating the data was retrieved directly from the buffer.
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| root@localhost:~# dd if=/dev/vda of=/dev/null bs=1M count=100
记录了100+0 的读入
记录了100+0 的写出
104857600字节（105 MB，100 MiB）已复制，0.0202984 s，5.2 GB/s
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Next, read a 33MB file using cat. As expected, the cache increased by 33MB.
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| root@localhost:~# ls -lh /var/log
......
-rw-r----- 1 xrdp adm 33M 11月 6 15:26 xrdp.log
......
root@localhost:~# cat /var/log/xrdp.log > /dev/null
root@localhost:~# cat /proc/meminfo |grep -E 'Buffers|Cache'
Buffers: 341648 kB
Cached: 1075616 kB
SwapCached: 88328 kB
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Use dd to write a 100MB file, then execute sync, while observing vmstat.
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| root@localhost:~# dd if=/dev/zero of=test.img bs=1M count=100
记录了100+0 的读入
记录了100+0 的写出
104857600字节（105 MB，100 MiB）已复制，0.0703571 s，1.5 GB/s
root@localhost:~# sync
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The dd command writes data very quickly, and the cache increased by 100MB, but the data wasn’t immediately written to disk. It was only after executing sync that the data was flushed to disk.
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| procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
r b 交换 空闲 缓冲 缓存 si so bi bo in cs us sy id wa st
0 0 214316 281576 338164 1019048 0 0 1 14 2 1 0 1 99 0 0
0 0 214316 281416 338164 1019060 0 0 0 0 961 286 0 0 100 0 0
0 0 214316 280808 338164 1019072 0 0 0 0 1111 294 0 1 99 0 0
0 0 214316 280232 338164 1019080 0 0 0 0 951 294 1 1 98 0 0
0 0 214316 280040 338164 1019088 0 0 0 0 765 279 1 0 99 0 0
0 0 214316 176712 338164 1121500 0 0 0 44 988 352 0 7 93 0 0
0 0 214316 176328 338164 1121508 0 0 0 300 1043 308 0 1 99 0 0
0 0 214316 176392 338164 1121520 0 0 0 8 960 314 1 0 99 0 0
0 0 214316 176296 338164 1121524 0 0 0 0 843 275 0 0 100 0 0
0 0 214316 175944 338164 1121532 0 0 0 124 682 355 0 1 98 1 0
0 0 214316 175616 338164 1121540 0 0 0 102760 1015 508 1 2 76 21 0
0 0 214316 175752 338164 1121556 0 0 0 0 971 343 0 1 99 0 0
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While writing to a block device with dd, the buffer also increases. Due to the lack of a proper test environment at hand, this part is not demonstrated (though it’s known that both buffer and cache can grow).