Raw and qcow2 disks are never preallocated on systems with newer util-linux

[Matthew Booth]

imagebackend.Image._can_fallocate tests if fallocate works by running the following command:

  fallocate -n -l 1 <image path>.fallocate_test

where <image path> exists, but <image path>.fallocate_test does not. This command line is copied from the code which actually fallocates a disk. However, while this works on systems with an older version of util-linux, such as RHEL 7, it does not work on systems with a newer version of util-linux, such as Fedora 23. The result of this is that this test will always fail, and preallocation with fallocate will be erroneously disabled.

On RHEL 7, which has util-linux-2.23.2-26.el7.x86_64 on my system:

$ fallocate -n -l 1 foo
$ ls -lh foo
-rw-r--r--. 1 mbooth mbooth 0 Feb 8 15:33 foo
$ du -sh foo
4.0K foo

On Fedora 23, which has util-linux-2.27.1-2.fc23.x86_64 on my system:

$ fallocate -n -l 1 foo
fallocate: cannot open foo: No such file or directory

The F23 behaviour actually makes sense. From the fallocate man page:

      -n, --keep-size
              Do not modify the apparent length of the file.

This doesn't make any sense if the file doesn't exist. That is, the -n option makes sense when preallocating an existing disk image, but not when testing if fallocate works on a given filesystem and the test file doesn't already exist.

You could also reasonably argue that util-linux probably should be breaking an interface like this, even when misused. However, that's a separate discussion. We shouldn't be misusing it.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/277402
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=33749d2875b4e63ad8f663a734cd087980489b6e
Submitter: Jenkins
Branch: master

commit 33749d2875b4e63ad8f663a734cd087980489b6e
Author: Matthew Booth <email address hidden>
Date: Mon Feb 8 12:41:06 2016 +0000

    Fix fallocate test on newer util-linux

    Newer util-linux raises an error when calling fallocate with the -n
    option if the target file does not already exist. This is because the
    -n option directs it to retain the file's existing size. A
    non-existent file does not have an existing size. fallocate in older
    releases of util-linux creates a zero-sized file in this case. This
    results in _can_fallocate() always returning false, and therefore
    never preallocating.

    While this may reasonably be argued to be a regression in util-linux,
    the -n option doesn't make sense here anyway, so we remove it.

    Closes-Bug: #1543181

    Change-Id: Ie96fa71e7d2641d30572b8eda5609dd3ca5b6708

[Doug Hellmann (doug-hellmann)]

This issue was fixed in the openstack/nova 13.0.0.0b3 development milestone.