Re: Disk I/O and memory/swapping

From: Jim85CJ (jim_85cj_at_NOSPAMyahoo.com)
Date: 06/11/04

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    Date: Fri, 11 Jun 2004 14:45:42 GMT

    Isn't using system cache for I/Os risky? If the system crashes and a
    couple of MB of I/Os are in memory (but ACKd back to the application)
    that data is lost and the application now has corrupted data. I don't
    think I've seen any other OS do it this way.

    Holger van Koll wrote:

    > Jim85CJ wrote:
    >
    >> I'm running an AIX 5.1 system with TSM. The memory always seems to be
    >> 100% utilized.
    >
    > Thats fine. (no matter what app you run)
    > unused memory is wasted money
    >
    >> When I run a large I/O (started using "dd" to test this to bypass the
    >> file system)
    >
    >
    > Please tell us the exact command. do you
    > dd <somewhere >/filesystem/file
    > or
    > dd </dev/hdisk# >/dev/null
    >
    >
    >> I see that the swap area start going nuts and I/Os to disk crawl. If
    >> I cancel the "dd" then I notice that the I/Os start hauling ass. Why
    >> is AIX sending disk I/Os to memory instead of going directly to disk?
    >
    >
    > To minimize head-movement -> to gain performance.
    > AIX keeps the data in ram until sync-daemon runs or other requirements
    > are met.
    >
    >
    >> The disk subsystem is attached to an IBM FC card. The disk subsystem
    >> shows minimal I/Os until I cancel the "dd"; then is starts cranking as
    >> AIX commits the cache to the disk array. How do I get AIX to write
    >> directly to disk?
    >
    > I dont see a way to do this on system-side.
    > you could run sync-daemon every 5 seconds, f.e. but i doubt your
    > performance will improve - it will probably suffer
    > you could also decrease numperm with vmo - but that will have the same
    > effects
    >
    > your application can tell aix to bypass memory - it has to open the file
    > with O_SYNC . this actually does not mean "bypass memory" but it comes
    > close

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