ATA 4K sector issues

[Matthew Dillon]

    I'll note one last thing with regards to write combining within the
    drive's zone cache.  Drive zone caches work very well for combing
    adjacent sectors when the write zones are perfectly linear (when
    the writes within each zone being tracked are perfectly linear).
    But the drive zone caches I've tested tend to break down very
    quickly when the data is written out of order, even if all the data winds
    up being present (could be combined into a linear result).

    I posted a program a while back that showed that.  Basically writing
    512-byte sectors ordered 0, 1, 2, 3, 4, ... is very fast, but the
    moment you start writing out of order, e.g. 3, 2, 1, 0, 7, 6, 5, 4,
    things start to break down very quickly.

    I would suspect the issue with UFS and a 2K fragment size on a 4K
    physical sector drive is precisely this.  When UFS mixes fragmented
    writes with full block writes (related to different files), they
    tend to have locality of reference but they also tend to be NOT
    perfectly linear.  2K+ gaps will be created during the unpack and
    filled later on, but the drive cache can't handle it.

    Having lots of directories with a few small files in each probably
    doesn't help matters any either but that can't be 'fixed' without
    messing up UFS's ability to maintain a relatively unfragmented
    filesystem over long periods of time.

    This is probably what is tripping the drive up.  This implies that
    you absolutely must use a 4K fragment size (32K block size) and an
    aligned partition when using UFS with 4K physical sector drives.

						-Matt