[erlang-questions] Linux O_DIRECT flag

[Valentin Micic]

Not when your application starts to swap-in and out because it is running out of RAM, and yet you still have plenty of RAM occupied by files that have been open and read, say few months ago, and no longer needed. Now, consider 64GB RAM of which 32 GB is occupied by cache. 

Between faster and fast enough, I'm always inclined to select fast enough, especially if that's going to allow me to use memory the way I need to vs. the way OS forces me to.

V.

  ----- Original Message ----- 
  From: Rapsey 
  To: erlang-questions@REDACTED 
  Sent: Friday, March 14, 2008 10:32 PM
  Subject: Re: [erlang-questions] Linux O_DIRECT flag


  I don't understand. Why is that a problem? Access to RAM will always be much faster than access to a storage array. 



  On Fri, Mar 14, 2008 at 8:45 PM, Valentin Micic <valentin@REDACTED> wrote:

    Dear all,

    One thing that has to make a top 10 most-likely-to-frustrate-you features in
    LINUX is its uncontrollable usage of memory for I/O caching. Whilst good for
    most applications, it seems quite redundant if you connect a big storage
    array that has a cache on its own.
    After a brief investigation regarding ones ability to bypass the cache
    (performed by our hardware supplier), I've been told that an application may
    bypass the cache by porviding additional flag to "open" system call --
    O_DIRECT to be more specific. I assume that file:open/2 does not support
    this flag, so I'd like to ask, before we start hacking around:  is there an
    easy way to expose this flag, and would you consider it for inclusion in one
    of the next releases. Provided that O_DIRECT does what we think it does, of
    course.

    Thanks in advance,

    Valenitn.

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