Re: O_DIRECT

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

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?

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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