Re: O_DIRECT question

[linux-os \(Dick Johnson\)]

On Wed, 10 Jan 2007, Aubrey 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?
>
> Thanks a lot.
> -Aubrey
> -

I don't think O_DIRECT ever did what a lot of folks expect, i.e.,
write this buffer of data to the physical device _now_. All I/O
ends up being buffered. The `man` page states that the I/O will
be synchronous, that at the conclusion of the call, data will have
been transferred. However, the data written probably will not be
in the physical device, perhaps only in a DMA-able buffer with
a promise to get it to the SCSI device, soon.

Maybe you need to say why you want to use O_DIRECT with its terrible
performance?

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.72 BogoMips).
New book: http://www.AbominableFirebug.com/
_


****************************************************************
The information transmitted in this message is confidential and may be privileged.  Any review, retransmission, dissemination, or other use of this information by persons or entities other than the intended recipient is prohibited.  If you are not the intended recipient, please notify Analogic Corporation immediately - by replying to this message or by sending an email to DeliveryErrors@xxxxxxxxxxxx - and destroy all copies of this information, including any attachments, without reading or disclosing them.

Thank you.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/