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    SubjectRe: O_DIRECT question
    On Saturday 27 January 2007 15:01, Bodo Eggert wrote:
    > Denis Vlasenko <> wrote:
    > > On Friday 26 January 2007 19:23, Bill Davidsen wrote:
    > >> Denis Vlasenko wrote:
    > >> > On Thursday 25 January 2007 21:45, Michael Tokarev wrote:
    > >> >> But even single-threaded I/O but in large quantities benefits from
    > >> >> O_DIRECT significantly, and I pointed this out before.
    > >> >
    > >> > Which shouldn't be true. There is no fundamental reason why
    > >> > ordinary writes should be slower than O_DIRECT.
    > >> >
    > >> Other than the copy to buffer taking CPU and memory resources.
    > >
    > > It is not required by any standard that I know. Kernel can be smarter
    > > and avoid that if it can.
    > The kernel can also solve the halting problem if it can.
    > Do you really think an entropy estamination code on all access patterns in the
    > system will be free as in beer,

    Actually I think we need this heuristic:

    if (opened_with_O_STREAM && buffer_is_aligned
    && io_size_is_a_multiple_of_sectorsize)

    is not *that* compilcated.

    I think that we can get rid of O_DIRECT peculiar requirements
    "you *must* not cache me" + "you *must* write me directly to bare metal"
    by replacing it with O_STREAM ("*advice* to not cache me") + O_SYNC
    ("write() should return only when data is written to storage, not sooner").


    Because these O_DIRECT "musts" are rather unusual and overkill. Apps
    should not have that much control over what kernel does internally;
    and also O_DIRECT was mixing shampoo and conditioner on one bottle
    (no-cache and sync writes) - bad API.
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      Last update: 2007-01-27 15:19    [W:0.022 / U:18.476 seconds]
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