O_DIRECT for block device

O_DIRECT for block device

[Karthik Sarangan]

I have a scsi hdd configured at /dev/sdb.

open("/dev/sdb", O_DIRECT) succeeds.
A 'read' or 'write' returns -1 and errno is EINVAL.

How to read and write data to it? They do not seem to work. I have a 
262144 byte buffer for data transfer.

My kernel is 2.6.9-11 and I use RedHat Enterprise Linux.

Re: O_DIRECT for block device

[Avi Kivity]

Karthik Sarangan wrote:

> I have a scsi hdd configured at /dev/sdb.
>
> open("/dev/sdb", O_DIRECT) succeeds.
> A 'read' or 'write' returns -1 and errno is EINVAL.
>
> How to read and write data to it? They do not seem to work. I have a 
> 262144 byte buffer for data transfer.

make sure your buffer is aligned on a 512 byte boundary.

Re: O_DIRECT for block device

[Joel Becker]

On Thu, Sep 22, 2005 at 01:51:16PM +0530, Karthik Sarangan wrote:
> open("/dev/sdb", O_DIRECT) succeeds.
> A 'read' or 'write' returns -1 and errno is EINVAL.

	You must align your buffer to at least sector size.  See
posix_memalign(3).

-- 

Life's Little Instruction Book #30

	"Never buy a house without a fireplace."

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@oracle.com
Phone: (650) 506-8127

Re: O_DIRECT for block device

[Karthik Sarangan]

Joel Becker wrote:
> On Thu, Sep 22, 2005 at 04:59:20PM +0530, Karthik Sarangan wrote:
> 
>>I tried your tip and the read works beautifully but the write returns -1 
>>and errno = EBADF!
> 
> 
> 	Did you open with O_RDWR?
> 
> Joel
> 

Alright I didn't, though using O_DIRECT | O_RDWR the second time solved 
it :-) I finally got 69MBps read and 49 MBps writes on my SCSI disk.

Thanks for helping me out.