lkml.org 
[lkml]   [2015]   [May]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: xfs: does mkfs.xfs require fancy switches to get decent performance? (was Tux3 Report: How fast can we fsync?)
    On 05/13/2015 04:31 AM, Daniel Phillips wrote:
    Let me be the first to catch that arithmetic error....

    > Let's say our delta size is 400MB (typical under load) and we leave
    > a "nice big gap" of 112 MB after flushing each one. Let's say we do
    > two thousand of those before deciding that we have enough information
    > available to switch to some smarter strategy. We used one GB of a
    > a 4TB disk, say. The media transfer rate decreased by a factor of:
    >
    > (1 - 2/1000) = .2%.

    Ahem, no, we used 1/8th of the disk. The time/data rate increased
    from unity to 1.125, for an average of 1.0625 across the region.
    If we only use 1/10th of the disk instead, by not leaving gaps,
    then the average time/data across the region is 1.05. The
    difference is, 1.0625 - 1.05, so the gap strategy increases media
    transfer time by 1.25%, which is not significant compared to the
    performance deficit in question of 400%. So, same argument:
    change in media transfer rate is just a distraction from the
    original question.

    In any case, we probably want to start using a smarter strategy
    sooner than 1000 commits, maybe after ten or a hundred commits,
    which would make the change in media transfer rate even less
    relevant.

    The thing is, when data first starts landing on media, we do not
    have much information about what the long term load will be. So
    just analyze the clues we have in the early commits and put those
    early deltas onto disk in the most efficient format, which for
    Tux3 seems to be linear per delta. There would be exceptions, but
    that is the common case.

    Then get smarter later. The intent is to get the best of both:
    early efficiency, and long term nice aging behavior. I do not
    accept the proposition that one must be sacrificed for the
    other, I find that reasoning faulty.

    > The performance deficit in question and the difference in media rate are
    > three orders of magnitude apart, does that justify the term "similar or
    > identical?".

    Regards,

    Daniel


    \
     
     \ /
      Last update: 2015-05-13 15:01    [W:4.867 / U:0.248 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site