Re: O_DIRECT

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Thank you very much ....
-Shesha

Rob van Nieuwkerk wrote:

On Wed, 21 Jul 2004 11:01:20 -0700
Shesha Sreenivasamurthy <shesha@xxxxxxxxxxx> wrote:

Hi Shesha,



ohhh OK, if the block size is 4096, then the read/write size must be integer multiple of 4096 ??? is it ???
In general should the read/write length be a multiple of block size?



Yes, see my previous emails.

	greetings,
	Rob van Nieuwkerk



Rob van Nieuwkerk wrote:



On Wed, 21 Jul 2004 10:10:26 -0700
Shesha Sreenivasamurthy <shesha@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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

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Kernelnewbies: Help each other learn about the Linux kernel.
Archive:       http://mail.nl.linux.org/kernelnewbies/
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Archive:       http://mail.nl.linux.org/kernelnewbies/
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