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    SubjectRe: BUG: jbd2 slowing down file copies even though no journaling file system is used
    From
    Ted,

    dm-1 points to my Linux-OS partition which is actually the encrypted
    LVM one. However, I could also see that on some copies this actually
    points to the related truecrypt partition so that makes sense there
    (the dm-1 doesn't imo but not important at this point).

    "echo 1 > /sys/kernel/debug/tracing/events/ext4/ext4_sync_file_enter/enable"
    didn't work for me, I got a "permission denied" error. So then I
    figured out I have to use "sudo -i" to get a root shell, then I can
    actually echo into that file ? Very strange but an Ubuntu
    issue/feature I guess.

    However, I don't see that the file /sys/kernel/debug/tracing/trace
    changes a lot? All I see is this even though I am clearly copying data
    onto the disk via samba:

    # tracer: nop
    #
    # TASK-PID CPU# TIMESTAMP FUNCTION
    # | | | | |
    <...>-2096 [001] 1111.151516: ext4_sync_file_enter: dev
    252,1 ino 3410181 parent 3410196 datasync 1
    <...>-2096 [000] 1111.201520: ext4_sync_file_enter: dev
    252,1 ino 3410181 parent 3410196 datasync 0
    <...>-2096 [001] 1111.252939: ext4_sync_file_enter: dev
    252,1 ino 3410181 parent 3410196 datasync 1
    <...>-2096 [000] 1111.290241: ext4_sync_file_enter: dev
    252,1 ino 3410181 parent 3410196 datasync 0
    <...>-2096 [001] 1111.341757: ext4_sync_file_enter: dev
    252,1 ino 3410181 parent 3410196 datasync 1
    <...>-2096 [000] 1111.367540: ext4_sync_file_enter: dev
    252,1 ino 3410181 parent 3410196 datasync 0
    <...>-2096 [001] 1111.419511: ext4_sync_file_enter: dev
    252,1 ino 3410181 parent 3410196 datasync 1
    <...>-2096 [000] 1111.456354: ext4_sync_file_enter: dev
    252,1 ino 3410181 parent 3410196 datasync 0

    PID 2096 is sshd acc. to "top" (I think this was my local copying to a
    file from /dev/zero?).

    Even if I copy data now, the file never changes, I always see the same
    "cat" output?

    I changed the entry back to 0, rebooted, changed it back to 1, started
    copying a file to an ext4 partition and cat
    /sys/kernel/debug/tracing/trace is just plain empty (shows "tracer:
    nop", TASK-PID etc but no data).

    I'm a bit lost on how to gather the required traces :(

    Thanks and regards,
    Bjoern

    2012/5/13 Ted Ts'o <tytso@mit.edu>:
    > On Sun, May 13, 2012 at 01:21:00AM +0200, Björn Christoph wrote:
    >>
    >> IOTOP shows this in some cases, mostly jbd2 is not there:
    >>   PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND
    >>   416 be/3 root        0.00 B/s   97.65 M/s  0.00 % 13.74 % [jbd2/dm-1-8]
    >
    > Well, the file system in that has the huge write bandwidth is
    > whichever device is associated with device mapper device dm-1 (i.e.,
    > with a device mapper minor number of 1).  What does "ls -l
    > /dev/mapper" show you?
    >
    > Whatever file system is associated with it is clearly generating a lot
    > of journal activity.
    >
    > Something that may help in determining what process is generating all
    > of this journal activity (which is likely the result of something
    > calling fsync a lot) is to try this:
    >
    > echo 1 > /sys/kernel/debug/tracing/events/ext4/ext4_sync_file_enter/enable
    >
    > ... then wait for a minute or so, and then capture the output of:
    >
    > cat /sys/kernel/debug/tracing/trace
    >
    > See if that shows up anything useful.
    >
    > Regards,
    >
    >                                        - Ted
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