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    SubjectRe: [PATCH] 498+ days uptime
    >>>>> ">" == Eric W Biederman <> writes:

    >>>> No. Major performance problem.

    BH> Why?

    BH> Imagine an application which has most of the (index) file pages in memory
    BH> and many of the pages are dirty. bdflush will flush the pages regularly,
    BH> but the pages will get dirty immediately again.
    BH> If you can be sure, that the power cannot fail the performance should be
    BH> much better without bdflush, because kflushd has to write pages only if
    BH> the system is running low on memory...

    >> The performance improvement comes when looking for free memory. In
    >> most cases bdflush's slow but steady writing of pages keeps buffers
    >> clean. When the application wants more memory with bdflush in the
    >> background unsually the pages it needs will be clean (because the I/O
    >> started before the application needed it), so they can just be dropped
    >> out of memory. Relying on kflushd means nothing is written until an
    >> application needs the memory and then it must wait until something is
    >> written to disk, which is much slower.

    >> Further
    >> a) garanteeing no power failure is hard.

    Use and UPS and regularly flush/sync the primary data to disk from
    the application

    >> b) generally there is so much data on the disk you must write it
    >> sometime, because you can't hold it all in memory.

    only a question of how much RAM you can put in your PC

    >> c) I have trouble imagining a case where a small file would be rewritten
    >> continually.

    Not really small, but a database application may use btree based indexes,
    where many blocks will get dirty when inserting/deleting data. If you flush
    the dirty buffers and the next insertion dirty the same buffer(s) you have
    lost performance (Note: the btree based indexes are secondary data; you
    can rebuild it from scratch if the system fails)


    get my pgp key from a public keyserver (keyID=0x62446355)
    Bernhard Heidegger
    Hyperwave Software Research & Development
    Schloegelgasse 9/1, A-8010 Graz
    Voice: ++43/316/820918-25 Fax: ++43/316/820918-99

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