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NerdOfLinux
Asked: 2018-03-20 05:37:40 +0800 CST

fallocate vs dd 交换文件？

  • 772

我想知道创建交换文件与

fallocate -l 1G /swapfile

和

dd if=/dev/zero of=/swapfile bs=1024 count=1024

两者似乎都可以正常工作，但是一个比另一个有优势吗？

我在网上唯一能找到的是它fallocate不适用于所有文件系统。

swap dd

2 个回答

  1. Best Answer

    从手册页：mkswap

    Note  that  a  swap  file  must  not contain any holes.  Using cp(1) to
create the file is not acceptable.  Neither is use of  fallocate(1)  on
file  systems  that support preallocated files, such as XFS or ext4, or
on copy-on-write filesystems like btrfs.   It  is  recommended  to  use
dd(1)  and  /dev/zero in these cases.  Please read notes from swapon(8)
before adding a swap file to copy-on-write filesystems.

    从手册页：swapon

    You should not use swapon on a file with holes.  This can  be  seen  in
the system log as

      swapon: swapfile has holes.

The  swap file implementation in the kernel expects to be able to write
to the file directly, without the assistance of the  filesystem.   This
is  a problem on preallocated files (e.g.  fallocate(1)) on filesystems
like XFS or ext4, and on copy-on-write filesystems like btrfs.

    由此可见，虽然fallocate可能比 快dd，但它不适合创建交换文件并且不受交换相关工具的支持。

    • 35

  2. Fallocate 更快，来自fallocate 手册页：

    fallocate用于操作为文件分配的磁盘空间，以解除分配或预分配。 对于支持 fallocate 系统调用的文件系统，通过分配块并将它们标记为未初始化来快速完成预分配，不需要对数据块进行 IO。这比通过填充零来创建文件要快得多。

    • 4

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