[erlang-questions] Linux O_DIRECT flag

[Toby Thain]

On 14-Mar-08, at 5:14 PM, Valentin Micic wrote:

> Not when your application starts to swap-in and out because it is  
> running out of RAM,

That should never happen! Are you sure you've got the right analysis?

--Toby

> 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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