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    SubjectRe: Linux 2.6.29
    On Wed, Mar 25, 2009 at 01:37:44PM +0100, Jan Kara wrote:
    > > Also, we do have to reliably get a lock on the buffer when moving it
    > > between lists and inspecting its internal state. Otherwise a competing
    > > read from the underlying block device can trigger an assertion failure,
    > > and a competing write to the underlying block device can confuse ext3
    > > journalling state completely.
    >
    > I've looked at this a bit. I suppose you mean the contention arising from
    > us taking the buffer lock in do_get_write_access()? But it's not obvious
    > to me why we'd be contending there... We call this function only for
    > metadata buffers (unless in data=journal mode) so there isn't huge amount
    > of these blocks.

    There isn't a huge number of those blocks, but if inode #1220 was
    modified in the previous transaction which is now being committed, and
    we then need to modify and write out inode #1221 in the current
    contention, and they share the same inode table block, that would
    cause the contention. That probably doesn't happen that often in a
    synchronous code path, but it probably happens more often that you're
    thinking. I still think the fsync() problem is the much bigger deal,
    and solving the contention problem isn't going to solve the fsync()
    latency problem with ext3 data=ordered mode.

    > Also when I emailed with a few people about these sync problems, they
    > wrote that switching to data=writeback mode helps considerably so this
    > would indicate that handling of ordered mode data buffers is causing most
    > of the slowdown...

    Yes, but we need to be clear whether this was an fsync() problem or
    some other random delay problem. If it's the fsync() problem,
    obviously data=writeback will solve the fsync() latency delay problem.
    (As will using delayed allocation in ext4 or XFS.)

    - Ted


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