dd:如何计算最佳块大小? [英] dd: How to calculate optimal blocksize?

问题描述

运行dd时如何计算最佳块大小?我已经对其进行了一些研究，但没有发现任何暗示如何实现此目标的信息.

How do you calculate the optimal blocksize when running a dd? I've researched it a bit and I've not found anything suggesting how this would be accomplished.

我的印象是更大的块大小会导致更快的dd ...是真的吗?

I am under the impression that a larger blocksize would result in a quicker dd... is this true?

我要dd在运行带有4GB DDR3 1333mhz RAM的Intel Core i3的盒子上以7200rpm运行的两个相同的500gb Hitachi HDD，因此，我试图找出要使用的块大小. (我要从闪存驱动器启动Ubuntu 10.10 x86，然后从中运行它.)

I'm about to dd two identical 500gb Hitachi HDDs that run at 7200rpm on a box running an Intel Core i3 with 4GB DDR3 1333mhz RAM, so I'm trying to figure out what blocksize to use. (I'm going to be booting Ubuntu 10.10 x86 from a flash drive, and running it from that.)

推荐答案

最佳块大小取决于各种因素，包括操作系统(及其版本)以及所涉及的各种硬件总线和磁盘.几个类似Unix的系统(包括Linux和至少某些BSD版本)定义了struct stat中的st_blksize成员，该成员给出了内核认为的最佳块大小:

The optimal block size depends on various factors, including the operating system (and its version), and the various hardware buses and disks involved. Several Unix-like systems (including Linux and at least some flavors of BSD) define the st_blksize member in the struct stat that gives what the kernel thinks is the optimal block size:

#include <sys/stat.h>
#include <stdio.h>

int main(void)
{
    struct stat stats;

    if (!stat("/", &stats))
    {
        printf("%u\n", stats.st_blksize);
    }
}

最好的方法可能是进行实验:复制一个具有各种块大小和时间的千兆字节. (请记住在每次运行之前清除内核缓冲区高速缓存:echo 3 > /proc/sys/vm/drop_caches).

The best way may be to experiment: copy a gigabyte with various block sizes and time that. (Remember to clear kernel buffer caches before each run: echo 3 > /proc/sys/vm/drop_caches).

但是，根据经验，我发现足够大的块大小可以使dd发挥出色，例如64 KiB和1 MiB之间的差异较小，而4 KiB与64 KiB. (尽管，自从我这样做以来已经有一段时间了.我现在默认情况下使用兆字节，或者只是让dd选择大小.)

However, as a rule of thumb, I've found that a large enough block size lets dd do a good job, and the differences between, say, 64 KiB and 1 MiB are minor, compared to 4 KiB versus 64 KiB. (Though, admittedly, it's been a while since I did that. I use a mebibyte by default now, or just let dd pick the size.)

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