dd 常用命令汇总





  1. dd bs=1M count=256 if=/dev/zero of=test
    The default behaviour of dd is to not “sync” (i.e. not ask the OS to completely write the data to disk before dd exiting). The above command will just commit your 256 MB of data into a RAM buffer (write cache) – this will be really fast and it will show you the hugely inflated benchmark result right away. However, the server in the background is still busy, continuing to write out data from the RAM cache to disk.
    dd 的默认行为是不做任何“同步”操作(也就是说,不要求操作系统在 dd 终止前完整的将数据写入磁盘。)上面那个命令只会给你分配 256M 的 RAM 缓存(写缓存)- 这个操作是非常快的,会告诉你非常夸张的测试觉果。但是实际上,服务器本身还在后台繁忙的进行写入操作,来将你刚刚写入 RAM 缓存的数据同步到磁盘。
  2. dd bs=1M count=256 if=/dev/zero of=test; sync
    Absolutely identical to the previous case, as anyone who understands how *nix shell works should surely know that adding a ; sync does not affect the operation of previous command in any way, because it is executed independently, after the first command completes. So your (inflated) MB/sec value is already printed on screen while that sync is only preparing to be executed.
    跟上一个命令完全相同。任何知道 *nix shell 如何运作的人都一定知道只是加一个 ; sync 并不会影响签名一条命令,因为这两条命令是完全分开执行的,添加的 sync 是在 dd 执行结束之后才会执行的。所以当你看到夸张的 MB/s 的值的时候,sync 命令才刚刚开始准备执行。
  3. dd bs=1M count=256 if=/dev/zero of=test conv=fdatasync
    This tells dd to require a complete “sync” once, right before it exits. So it commits the whole 256 MB of data, then tells the operating system: “OK, now ensure this is completely on disk”, only then measures the total time it took to do all that and calculates the benchmark result.
    这条指令告诉 dd 在退出之前必须要等待“同步”结束。所以该命令先是创建了 256M 的数据,然后告诉系统:“现在确认这些数据已经写到了磁盘上”,然后才开始停止计时并计算测试的结果。
  4. dd bs=1M count=256 if=/dev/zero of=test oflag=dsync
    Here dd will ask for completely synchronous output to disk, i.e. ensure that its write requests don’t even return until the submitted data is on disk. In the above example, this will mean sync’ing once per megabyte, or 256 times in total. It will be the slowest mode, as the write cache is basically unused at all in this case.
    这条命令 dd 会要求完全同步写入磁盘,也就是说,任何一个写入请求在数据完全保存到磁盘之前都不会返回。在上面这条命令里,这代表 dd 将会每 1MB 同步一次,或者说一共要同步256次。所以这是最慢模式,因为这种情况下基本上完全没有使用缓存。

    当然有时候连续执行第三条和第四条命令,第四条并不比第三条慢。这可能是由于其他应用的 IO 占用导致的。



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