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    SubjectGFS2 file locking issues
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    Hi,

    I'm trying to understand how the FS locking mechanism should work.

    I'm running a clustered GFS2 across two nodes, each node is a Xen domU.
    To check if locking works correctly I wrote the simple perl script below.
    Basically the script opens a file, locks it to prevent others from
    writing into it, writes 20 lines into it, then unlocks and closes.

    ------------------------------------------------------------------------------
    #!/usr/bin/perl

    # usage:
    # lock.pl filename uniq
    # filename - file name being written to
    # uniq - an unique label to distinguish output of
    different processes

    use strict;
    use Time::HiRes qw(sleep);
    use FileHandle;
    use Fcntl ':flock';

    my $filename = $ARGV[0];
    my $uniq = $ARGV[1];

    open FH, '>>', $filename or die $!;
    flock(FH,LOCK_EX) or die $!;
    FH->autoflush(1);

    for (1..20) {
    print FH "$uniq\n";
    sleep (0.1);
    }

    flock(FH,LOCK_UN);
    close FH;
    ------------------------------------------------------------------------------

    I ran that script on both nodes simultaneously with commands:

    vm01# perl lock.pl /gfs2/testfile a
    vm02# perl lock.pl /gfs2/testfile b

    Then to my surprise "a"s and "b"s are randomly shuffled in the testfile like

    ...
    a
    a
    b
    a
    b
    b
    ...

    whereas I supposed it should have been like

    ...
    a
    a
    a
    b
    b
    b
    ...


    It looks like either locking is broken in my GFS2 (I use kernel
    2.6.18-92.el5xen for CentOS 5.2) or my understanding of locking isn't
    correct.
    I also tested the script on GFS1 and OCFS2 and it worked well, i.e.
    the output was correct.



    Thanks,
    Kirill


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