我正在编写具有一些实时约束的 C 代码。我测试了使用 dd 写入磁盘的速度：
dd if=/dev/zero of=/dev/sdb bs=32K count=32768 oflag=direct
这将以 32K 块大小将 1GB 的零写入 /dev/sdb
我达到了大约 103 MB/s
现在我以编程方式执行类似的操作：
open("/dev/sdb",O_WRONLY|O_CREAT|O_DIRECT|O_TRUNC, 0666);
我得到一个时间戳值
从 32K 缓冲区写入 /dev/sdb 10,000 次（在 for 循环中）
获取另一个时间戳值
进行一些数字运算以获得以 MB/s 为单位的速率，大约为 49 MB/s
为什么我不能达到与 dd 相同的速度？ strace 显示了我使用的相同打开命令。
I'm writing C code with some real-time constraints. I tested out the speed I can write to a disk with dd:
dd if=/dev/zero of=/dev/sdb bs=32K count=32768 oflag=direct
This writes 1GB of zeros to /dev/sdb in 32K block sizes
I reach about 103 MB/s with this
Now I programmatically do something similar:
open("/dev/sdb",O_WRONLY|O_CREAT|O_DIRECT|O_TRUNC, 0666);
I get a timestamp value
write from a 32K buffer to /dev/sdb 10,000 times (in a for loop)
get another timestamp value
do a bit of number crunching to get the rate in MB/s and it is about 49 MB/s
Why can't I reach the same speed as dd? An strace reveals the same open command that I use.
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我将把有关匹配系统调用的部分留给其他人。这个答案是关于缓冲部分的。
尝试对您使用的缓冲区大小进行基准测试。尝试一系列值。
在学习 Java 时，我编写了一个简单的“copy”克隆，然后尝试匹配它的速度。由于代码是逐字节读/写的，所以缓冲区大小才是真正造成差异的原因。我自己没有缓冲它，但我要求读取获取给定大小的块。块越大，速度就越快——直到一定程度。
至于使用 32K 块大小，请记住操作系统仍然为用户模式进程使用单独的 IO 缓冲区。即使您正在使用特定的硬件进行某些操作，即您正在为具有某些物理限制的设备（例如具有扇区大小的 CD-RW 驱动器）编写驱动程序，块大小也只是问题的一部分。操作系统仍然会有它的缓冲区。
I'm leaving the part about matching the system calls to somebody else. This answer is about the buffering part.
Try benchmarking the buffer size you use. Experiment with a range of values.
When learning Java, I wrote a simple clone of 'copy' and then tried to match it's speed. Since the code did byte-by-byte read/writes the buffer size was what really made the difference. I wasn't buffering it myself but I was asking the read to fetch chunks of a given size. The bigger the chunk, the faster it went - up to a point.
As for using 32K block size, remember that the OS still uses separate IO buffers for user-mode processes. Even if you are doing something with specific hardware, i.e. you're writing a driver for a device that has some physical limitation, e.g. a CD-RW drive with sector sizes, the block size is only part of the story. The OS will still have it's buffer too.
检查
dd进行了哪些系统调用，不仅是打开，还包括后续的read和writes。使用正确的缓冲区大小可以在这种大型副本中产生显着的差异。请注意，如果您的最终目标是磁盘到磁盘的复制，那么/dev/zero并不是一个很好的基准测试。如果您无法通过将系统调用与系统调用相匹配来匹配 dd 的速度...那么，请阅读源代码。
Check what system calls
ddmakes, not just the open but also the subsequentreads andwrites. Using the right buffer sizes can make a significant difference in this kind of large copy. Note that/dev/zerois not a good test for benchmarking if your final goal is a disk-to-disk copy.If you can't match
dd's speed by matching it system call for system call... well, read the source.