為什麼記憶體讀取比 hdparm –direct 中的磁碟讀取慢？

我試圖解釋 hdparm 的這個結果：

janus@behemoth ~ $ sudo hdparm  -Tt --direct /dev/nvme0n1

/dev/nvme0n1:
Timing O_DIRECT cached reads:   2548 MB in  2.00 seconds = 1273.69 MB/sec
Timing O_DIRECT disk reads: 4188 MB in  3.00 seconds = 1395.36 MB/sec

我不明白記憶體讀取如何比直接磁碟讀取慢。如果我放棄 –direct，我會得到我所期望的：磁碟讀取比記憶體的慢：

janus@behemoth ~ $ sudo hdparm  -Tt /dev/nvme0n1

/dev/nvme0n1:
Timing cached reads:   22064 MB in  2.00 seconds = 11042.86 MB/sec
Timing buffered disk reads: 2330 MB in  3.00 seconds = 776.06 MB/sec

（雖然它現在說“緩衝磁碟讀取”）。

有人可以向我解釋發生了什麼嗎？

每個hdparm手冊頁：

--direct

Use the kernel´s "O_DIRECT" flag when  performing  a  -t  timing
test.   This  bypasses  the  page cache, causing the reads to go
directly from the drive into hdparm's buffers,  using  so-called
"raw"  I/O.  In many cases, this can produce results that appear
much faster than the usual page cache method,  giving  a  better
indication of raw device and driver performance.

它如此解釋了為什麼hdparm -t --direct可能比hdparm -t. 它還說--direct僅適用於-t測試，不適用於-T不應該涉及磁碟的測試（見下文）。

-T 

Perform timings of cache reads for benchmark and comparison pur‐
poses.   For  meaningful  results,  this  operation  should   be  
repeated  2-3  times  on  an otherwise inactive system (no other
active processes) with at least a couple of  megabytes  of  free
memory.   This  displays  the speed of reading directly from the 
Linux buffer cache without disk  access.   This  measurement  is  
essentially  an  indication  of the throughput of the processor,
cache, and memory of the system under test.

我猜想-T通過讀取磁碟的相同記憶體部分來工作。但是你--direct阻止了這一點。所以，從邏輯上講，你應該得到與-t --directas相同的結果-T --direct。

引用自：https://unix.stackexchange.com/questions/383712