Re: O_DIRECT

[Shesha Sreenivasamurthy <shesha@inostor.com>]

Thank you very much ....
-Shesha

Rob van Nieuwkerk wrote:

>On Wed, 21 Jul 2004 11:01:20 -0700
>Shesha Sreenivasamurthy <shesha@inostor.com> 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@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/
>>>>>
>>>>>
>>>>>.
>>>>>
>>>>>
>>>>>
>>>>>     
>>>>>
>>>>>          
>>>>>
>>>--
>>>Kernelnewbies: Help each other learn about the Linux kernel.
>>>Archive:       http://mail.nl.linux.org/kernelnewbies/
>>>FAQ:           http://kernelnewbies.org/faq/
>>>
>>>
>>>.
>>>
>>> 
>>>
>>>      
>>>
>
>--
>Kernelnewbies: Help each other learn about the Linux kernel.
>Archive:       http://mail.nl.linux.org/kernelnewbies/
>FAQ:           http://kernelnewbies.org/faq/
>
>
>.
>
>  
>