09-28-2010
There a more drawbacks to doing that than there are advantages. Oracle and other high I/O demand products usually will work with raw devices to bypass the overhead of a filesystem. This means Oracle keeps track of everything on the disk, like the location of data items.
Basically you are also bypassing security and other featuresprovided by the kernel/filesystem to gain I/O speed.
All of your code has to run as root, so it means you can damage the kernel itself quite easily.
This is how to create raw devices for RedHat Linux that you access via your driver code:
How To Add Raw Device Mapping In Red Hat Enterprise Linux 5?
You can call open, close, read & write on a raw device fd you get from calling open. If you are running as root.
This User Gave Thanks to jim mcnamara For This Post:
8 More Discussions You Might Find Interesting
1. Solaris
Hi
I have solaris 8 installed on Intel machine. the disk I have is IDE.
I would like to know how can I create a raw partition on an IDE disk.
Regards,
Raja (2 Replies)
Discussion started by: RajaRC
2 Replies
2. UNIX for Advanced & Expert Users
Hello all,
I'm new here, so this information may exist elsewhere on this forum. If so, please point me in the right direction.
Here's the problem.
I'm trying to migrate Oracle data from an HP system to a Sun system using a raw device as a 'bridge' between the two systems. Both machines... (4 Replies)
Discussion started by: Neville
4 Replies
3. UNIX for Dummies Questions & Answers
What do u mean by raw and cooked disk? What are the advantages of having raw disk?
Thanks n regards, (1 Reply)
Discussion started by: kingsto88
1 Replies
4. AIX
Hi all,
We have a problem after exporting a exhanced concurrent capable VG accessed in concurrent mode, created on raw devices ( pSeries 590 AIX5.3 + HACMP5.3 +SAN4300) from node A to node B.
On node B the VG is correct activated by HACMP, but on node A we have 0516-306 error "Unable to find... (3 Replies)
Discussion started by: cris1357
3 Replies
5. AIX
Is there any system call available in AIX to read the size of raw disk?
If I use the command "lspv -L",it only gives size of PVs on which file system is there. I need to extract the size raw disk i.e. file system is not there on the disk.
Thanks,
Megha (6 Replies)
Discussion started by: MeghaV
6 Replies
6. UNIX for Dummies Questions & Answers
I am having trouble understanding the difference between a passthrough device and a named device and when you would use one or the other to access equipment.
As an example, we have a tape library and giving the command
"camcontrol devlist" gives the following output:
akx# camcontrol... (1 Reply)
Discussion started by: thumper
1 Replies
7. Filesystems, Disks and Memory
The query is as follows :
A typical server configs when using Oracle or any other type of DB is to install the OS + DB binaries on the internal disks of the relevant server e.g.
Disk 1 : OS + SW + DB binaries
Disk 2 : Mirror of disk 1 (used for resiliency)
Then one uses an external array... (1 Reply)
Discussion started by: kekanap
1 Replies
8. Red Hat
Can you please modify my script. This script is not working
for i in /dev/sdf
do
/bin/raw /dev/raw/`/bin/basename ${i}` ${i}
/bin/sleep 2
/bin/chown orasm:ordba /dev/raw/`/bin/basename ${i}`
/bin/chmod 660... (9 Replies)
Discussion started by: karthik9358
9 Replies
RAW(8) System Administration RAW(8)
NAME
raw - bind a Linux raw character device
SYNOPSIS
raw /dev/raw/raw<N> <major> <minor>
raw /dev/raw/raw<N> /dev/<blockdev>
raw -q /dev/raw/raw<N>
raw -qa
DESCRIPTION
raw is used to bind a Linux raw character device to a block device. Any block device may be used: at the time of binding, the device
driver does not even have to be accessible (it may be loaded on demand as a kernel module later).
raw is used in two modes: it either sets raw device bindings, or it queries existing bindings. When setting a raw device, /dev/raw/raw<N>
is the device name of an existing raw device node in the filesystem. The block device to which it is to be bound can be specified either
in terms of its major and minor device numbers, or as a path name /dev/<blockdev> to an existing block device file.
The bindings already in existence can be queried with the -q option, which is used either with a raw device filename to query that one
device, or with the -a option to query all bound raw devices.
Unbinding can be done by specifying major and minor 0.
Once bound to a block device, a raw device can be opened, read and written, just like the block device it is bound to. However, the raw
device does not behave exactly like the block device. In particular, access to the raw device bypasses the kernel's block buffer cache
entirely: all I/O is done directly to and from the address space of the process performing the I/O. If the underlying block device driver
can support DMA, then no data copying at all is required to complete the I/O.
Because raw I/O involves direct hardware access to a process's memory, a few extra restrictions must be observed. All I/Os must be cor-
rectly aligned in memory and on disk: they must start at a sector offset on disk, they must be an exact number of sectors long, and the
data buffer in virtual memory must also be aligned to a multiple of the sector size. The sector size is 512 bytes for most devices.
OPTIONS
-q, --query
Set query mode. raw will query an existing binding instead of setting a new one.
-a, --all
With -q , specify that all bound raw devices should be queried.
-h, --help
Display help text and exit.
-V, --version
Display version information and exit.
BUGS
The Linux dd(1) command should be used without the bs= option, or the blocksize needs to be a multiple of the sector size of the device
(512 bytes usually), otherwise it will fail with "Invalid Argument" messages (EINVAL).
Raw I/O devices do not maintain cache coherency with the Linux block device buffer cache. If you use raw I/O to overwrite data already in
the buffer cache, the buffer cache will no longer correspond to the contents of the actual storage device underneath. This is deliberate,
but is regarded either a bug or a feature depending on who you ask!
NOTES
Rather than using raw devices applications should prefer open(2) devices, such as /dev/sda1, with the O_DIRECT flag.
AUTHOR
Stephen Tweedie (sct@redhat.com)
AVAILABILITY
The raw command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/.
util-linux August 1999 RAW(8)