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    SubjectRe: Performance of ext4
    On Thu, 19 Jun 2008, Theodore Tso wrote:

    > On Thu, Jun 19, 2008 at 11:41:17AM -0500, Eric Sandeen wrote:
    >> It might be worth runninga "simple" fsx under your kernel too; last time
    >> I tested fsx it was still happy and it exercises fs ops (including
    >> truncate) at random...
    > From what Holger described, it's doubtful that the bug is in the
    > truncate operation.
    Correct, the benchmark just copies, moves, hardlinks and deletes a lot
    of small files. It also overwrites existing files but not at the same
    scale it does the other operations.

    > It sounds like i_size is actually dropping in
    > size at some pointer long after the file was written. If I had to
    > guess the value in the inode cache is correct; and perhaps so is the
    > value on the journal. But somehow, the wrong value is getting written
    > to disk (remember the jbd layer can keep up to three different
    > versions of filesystem metadata in memory, because most of the time we
    > don't block modifications to the filesystem while we are in the middle
    > of writing a previous commit to disk). So depending on whether the
    > inode gets redirtied or not, the inconsistency could self-heal, and if
    > the inode never gets pushed out of memory due to memory pressure, the
    > problem might not be noticed until the system reboots or the
    > filesystem is unmounted.
    I always had the feeling that waiting a day or unmounting caused a lot
    more truncation. On my system at home for example I mounted the test
    filesystem again and saw that files where truncated and I am pretty sure
    that when I looked at those files during and shortly after the test they
    where still complete. But I will recheck and do test as you suggested.

    What I find strange is that the missing parts of the file are not for
    example exactly 512 or 1024 or 4096 bytes it is mostly some odd number
    of bytes.


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