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SubjectRe: O_DIRECT
On Wed, 21 Jul 2004 10:10:26 -0700
Shesha Sreenivasamurthy <shesha@inostor.com> wrote:

Hi Shesha,

You don't mention what the *size* of your read()/write() is.
Besides a requirement on the alignment of the read/write buffer
the size of the read()/write() must also be OK.

greetings,
Rob van Nieuwkerk

> This is what I found ....
>
> Our driver sets the block size to be 4096. so BLKBSZGET will return
> 4096. So if I allin the memory at 4096 boundary, I cannot read using
> O_DIRECT. But, if I set the block size to 512. I can read/write
> successfully. It also works with 1024, but no with 4096
>
> So the recepie what I am following is ...
>
> BLKBSZGET -> Get original block size
> BLKBSZSET -> Set the block size to 512
> READ | WRITE Successfully ;)
> BLKBSZSET -> Set back to the original block size
>
> -Shesha
>
> Rob van Nieuwkerk wrote:
>
> >On Tue, 20 Jul 2004 10:27:57 -0700
> >Shesha Sreenivasamurthy <shesha@inostor.com> wrote:
> >
> >Hi Shesha,
> >
> >
> >
> >>I am having trouble with O_DIRECT. Trying to read or write from a block
> >>device partition.
> >>
> >>1. Can O_DIRECT be used on a plain block device partition say
> >>"/dev/sda11" without having a filesystem on it.
> >>
> >>
> >
> >yes.
> >
> >
> >
> >>2. If no file system is created then what should be the softblock size.
> >>I am using the IOCTL "BLKBSZGET". Is this correct?
> >>
> >>
> >
> >yes.
> >
> >
> >
> >>3. Can we use SEEK_END with O_DIRECT on a partition without filesystem.
> >>
> >>
> >
> >yes.
> >
> >I'm using these exact things in an application.
> >
> >Note that with 2.4 kernels the "granularity" you can use for offset
> >and r/w size is the softblock size (*). For 2.6 the requirements are
> >much more relaxed: it's the device blocksize (typically 512 byte).
> >
> >(*): actually one of offset or r/w size has a smaller minimum if
> >I remember correctly. Don't remember which one. But if you assume
> >the softblock size as a minimum for both you're allways safe.
> >
> > greetings,
> > Rob van Nieuwkerk
> >
> >--
> >Kernelnewbies: Help each other learn about the Linux kernel.
> >Archive: http://mail.nl.linux.org/kernelnewbies/
> >FAQ: http://kernelnewbies.org/faq/
> >
> >
> >.
> >
> >
> >
>
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