Re: O_DIRECT question

[Andrew Morton]

On Thu, 11 Jan 2007 10:57:06 +0800
Aubrey <aubreylee@xxxxxxxxx> wrote:

> Hi all,
> 
> Opening file with O_DIRECT flag can do the un-buffered read/write access.
> So if I need un-buffered access, I have to change all of my
> applications to add this flag. What's more, Some scripts like "cp
> oldfile newfile" still use pagecache and buffer.
> Now, my question is, is there a existing way to mount a filesystem
> with O_DIRECT flag? so that I don't need to change anything in my
> system. If there is no option so far, What is the right way to achieve
> my purpose?

Not possible, basically.

O_DIRECT reads and writes must be aligned to the device's block size
(usually 512 bytes) in memory addresses, file offsets and read/write request
sizes.  Very few applications will bother to do that and will hence fail if
their files are automagically opened with O_DIRECT.

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