Re: DIRECT IO for ext3/ext2.

[Andrew Morton <akpm@osdl.org>]

<dhruv.anand@wipro.com> wrote:
>
> 
> Hi,
> I am working on an application on linux-2.6 that needs to
> bypass the buffer cache. In order to do so i use the direct
> IO functionality. Although open to the device succeeds with
> the DIRECT_IO flag, read from the device fails.
> 
> Following is the exceprt fromt he code to open and read;
> --------------------------------------------------------
> 
> if ((devf = open(dumpdev, O_RDONLY | O_DIRECT, 0)) < 0) {
>      fprintf(KL_ERRORFP, "Error: open failed!\n");
>      ...
> }
> 
> if(err = read(devf, &magic_nr, sizeof(magic_nr)) != sizeof(magic_nr)) {
>      fprintf(KL_ERRORFP, "Error: read() failed!\n");
>       ...
> }
> 
> ---------------------------------------------------------
> I am returned an errno=22, indicating 'Invalid argument'
> 

O_DIRECT reads must be aligned to the filesystem blocksize.  Both the
memory address and the file offset must be thus aligned.