raw I/O support for Fedora Core 4

raw I/O support for Fedora Core 4

[Yogesh Pahilwan]

Hi All,

I want to do raw I/O on MD RAID and LVM for fedora core 4(kernel 2.6.15.6).

After doing googling I came to know that "raw" command does the raw
operation by linking 
MD device and LVM volume to the raw device as 

# raw /dev/raw/raw1 /dev/md0.

But when I search on this I came to know that there is no raw (/dev/rawctl)
device support available with 2.6 kernel.
I have also tried recompile the kernel sources with raw device support it is
not getting compiled as it is obsolete in 2.6. 
If I want to include raw device support in my kernel what should I will have
to do, so that I 
Will be able to do raw I/O on MD device and LVM volumes.

Thanks and Regards,
Yogesh

Re: raw I/O support for Fedora Core 4

[Arjan van de Ven]

On Thu, 2006-03-23 at 14:54 +0530, Yogesh Pahilwan wrote:
> Hi All,
> 
> I want to do raw I/O on MD RAID and LVM for fedora core 4(kernel 2.6.15.6).
> 
> After doing googling I came to know that "raw" command does the raw
> operation by linking 
> MD device and LVM volume to the raw device as 
> 
> # raw /dev/raw/raw1 /dev/md0.
> 
> But when I search on this I came to know that there is no raw (/dev/rawctl)
> device support available with 2.6 kernel.
> I have also tried recompile the kernel sources with raw device support it is
> not getting compiled as it is obsolete in 2.6. 
> If I want to include raw device support in my kernel what should I will have
> to do, so that I 

the preferred interface is doing O_DIRECT io on the device node itself,
rather than going via a (slower) indirection layer such as "raw". ("raw"
is just a wrapper on top of O_DIRECT basically in 2.6 kernels so slower)

Re: raw I/O support for Fedora Core 4

[Phillip Susi]

The raw device driver is obsolete because it has been superseded by the 
O_DIRECT open flag.  If you want to have dd perform unbuffered IO then 
pass the iflag=direct option for input, or oflag=direct option for 
output, and it will use O_DIRECT to bypass the buffer cache.

This of course assumes that you mean "bypass the buffer cache" when you 
say "raw io".

Yogesh Pahilwan wrote:
> Hi All,
> 
> I want to do raw I/O on MD RAID and LVM for fedora core 4(kernel 2.6.15.6).
> 
> After doing googling I came to know that "raw" command does the raw
> operation by linking 
> MD device and LVM volume to the raw device as 
> 
> # raw /dev/raw/raw1 /dev/md0.
> 
> But when I search on this I came to know that there is no raw (/dev/rawctl)
> device support available with 2.6 kernel.
> I have also tried recompile the kernel sources with raw device support it is
> not getting compiled as it is obsolete in 2.6. 
> If I want to include raw device support in my kernel what should I will have
> to do, so that I 
> Will be able to do raw I/O on MD device and LVM volumes.
> 
> Thanks and Regards,
> Yogesh
>