Re: O_DIRECT

[Shesha Sreenivasamurthy <shesha@inostor.com>]

You are right I am not using any file system. Just did that experiment 
by creating FS too. That also did not help. I shared that information. 
You are absolutely right I am dealing with straight partitions.

(1) With staraight partition even if I explicitly set the block size to 
4096 and adher to the rules of O_DIRECT i cannot read/write

(2) With staraight partition if I set the block size to 1024 and adher 
to the rules of O_DIRECT i can read/write

Ruls include : Alligning the buffer & read/write multiple of block size.

-Shesha

Rob van Nieuwkerk wrote:

>On Wed, 21 Jul 2004 16:18:37 -0700
>Shesha Sreenivasamurthy <shesha@inostor.com> wrote:
>
>Hi Shesha,
>
>Your original question (and my answer) was about a different situation:
>
>        - partition with no filesystem
>        - access on this raw partition
>
>Now you start talking about various file-systems and I get the impression
>that you access the device through the file-system (although that is
>not very clear in your text) on a mounted fs.
>
>What I told you applied to straight partitions (without mounted a mounted fs).
>
>I don't know the exact rules for the various file-systems.
>They might be different (and some might not support O_DIRECT at all).
>I expect the same "soft block size rules" to apply for the filesystems
>that *do* support O_DIRECT.  Note that mounting a partition might
>change the softblock size of that partition used by the kernel from thereon.
>
>
>	greetings,
>	Rob van Nieuwkerk
>
>PS1: what kernel are you using ?
>
>PS2: what are you trying to do ?
>
>PS3: you mention "our driver ..".  What is this driver ?
>
>
>
>  
>
>>I could never get it working with 4096 block size.
>>
>>I created an EXT3 with blockize=4096 (mkfs.ext3 -b 4096 /dev/sda11). 
>>Alligned memory to 4096 boundary. Read 8192 Bytes. -> UNSUCCESSFUL :(
>>I created an XFS with blockize=4096 (mkfs.xfs -f  -b size=4096 
>>/dev/sda11). Alligned memory to 4096 boundary. Read 8192 Bytes. -> 
>>UNSUCCESSFUL :(
>>
>>SUCCESS HAPPEND ONLY WHEN ....
>>
>>1. I created an XFS with blockize=4096 (mkfs.xfs -f  -b size=4096 
>>/dev/sda11).
>>                                                       OR
>>    I created an EXT3 with blockize=4096 (mkfs.ext3 -b 4096 /dev/sda11).
>>
>>2. Changed blocksizeto 1024
>>3. Alligned memory to 1024 boundary.
>>4  Read 8192 Bytes. -> SUCCESSFUL :)
>>
>>-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/
>>>
>>>
>>>.
>>>
>>> 
>>>
>>>      
>>>
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