RE: DIRECT IO for ext3/ext2.

RE: DIRECT IO for ext3/ext2.

[dhruv.anand]

Hi Andrew,

Thanks for your reply. I looked into the boundary alignment issue. 
Observed something strange happening;

I wrote my own kernel module and then;
1. retrieved the block size = 4096 using block_size(struct block_device
*bdev) 
2. in the user application i did a lseek by 4906 (block size seek) 
3. then found the address of the variable i wanted to read into, to be
block 
   size aligned.

However, the read still seems to fail with the O_DIRECT flag specified. 
The same check _passes if i do not specify the flag in open of the
device.

Boundary issues seem to be not the cause of failure here.
Would have any more kind suggestions on something that i should do or
could be missing on?

Regards
Dhruv

>>-----Original Message-----
>>From: Andrew Morton [mailto:akpm@osdl.org] 
>>Sent: Friday, November 21, 2003 11:11 PM
>>To: Dhruv Prem Anand (WT01 - EMBEDDED & PRODUCT ENGINEERING SOLUTIONS)
>>Cc: linux-kernel@vger.kernel.org; akpm@zip.com.au; 
>>janetinc@us.ibm.com; pbadari@us.ibm.com; nathans@sgi.com
>>Subject: Re: DIRECT IO for ext3/ext2.
>>
>>
>><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.
>>
>>

Re: DIRECT IO for ext3/ext2.

[Andrew Morton]

<dhruv.anand@wipro.com> wrote:
>
> Thanks for your reply. I looked into the boundary alignment issue. 
>  Observed something strange happening;
> 
>  I wrote my own kernel module and then;
>  1. retrieved the block size = 4096 using block_size(struct block_device
>  *bdev) 
>  2. in the user application i did a lseek by 4906 (block size seek) 
>  3. then found the address of the variable i wanted to read into, to be
>  block 
>     size aligned.

O_DIRECT works - trust me ;)

Please grab

	http://www.zip.com.au/~akpm/linux/patches/stuff/ext3-tools.tar.gz

There are a number of examples in there (odread.c is one).

Re: DIRECT IO for ext3/ext2.

[Michael Kerrisk]

> Thanks for your reply. I looked into the boundary alignment issue.
> Observed something strange happening;
>
> I wrote my own kernel module and then;
> 1. retrieved the block size = 4096 using block_size(struct block_device
> *bdev)
> 2. in the user application i did a lseek by 4906 (block size seek)
> 3. then found the address of the variable i wanted to read into, to be
> block
>    size aligned.
>
> However, the read still seems to fail with the O_DIRECT flag specified.
> The same check _passes if i do not specify the flag in open of the
> device.
>
> Boundary issues seem to be not the cause of failure here.
> Would have any more kind suggestions on something that i should do or
> could be missing on?

Is the size of the data you are reading also a multiple the block size?  (It
should be.)

Cheers,

Michael

DIRECT IO for ext3/ext2.

[dhruv.anand]

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'

I would appreciate it, if you could knowledge me importantly about
the completeness of the 'direct IO' functionality in ext3 file-system.
Or if i am doing something wrong in usage?


Regards
Dhruv.

Re: DIRECT IO for ext3/ext2.

[Andrew Morton]

<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.