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

fallocate vs dd for swapfile?

  • 772

I'm wondering what the difference is between creating a swapfile with 

fallocate -l 1G /swapfile

and 

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

both seem to work fine, but does one have an advantage over the other?

The only thing I could find online was that fallocate does not work on all file systems.

swap dd

2 Answers

  1. Best Answer

    From the mkswap manpage:

    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.

    And from the swapon manpage:

    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.

    It follows that, while fallocate may be faster than dd, it's not suitable for creating swap files and not supported by swap-related tools.

    • 35

  2. Fallocate is faster, From the fallocate manpage:

    fallocate is used to manipulate the allocated disk space for a file, either to deallocate or preallocate it. For filesystems which support the fallocate system call, preallocation is done quickly by allocating blocks and marking them as uninitialized, requiring no IO to the data blocks. This is much faster than creating a file by filling it with zeros.

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