raw device

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FAQ Details
  

Q:  1. What is a raw device?

A:Raw device, also known as a raw partition is a disk partition that is not mounted and written by Linux filesystem (ext2/ext3, reiserfs) or by Oracle Cluster File System (OCFS), but is accessed by a character device driver. It is the responsibility of the application to organize how the data is written to the disk partition.

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Q:  2. How can a raw device be recognised?

A:All hardware devices look like regular files; they can be opened, closed, read and written using the same, standard, system calls that are used to manipulate files. Every device in the system is represented by a device special file, for example the first IDE disk in the system is represented by /dev/hda. For block (disk) and character devices, these device special files are created by the mknod command and they describe the device using major and minor device numbers.
All devices controlled by the same device driver have a common major device number.
The minor device numbers are used to distinguish between different devices and their controllers, for example each partition on the primary IDE disk has a different minor device number. So, /dev/hda2, the second partition of the primary IDE disk has a major number of 3 and a minor number of 2. Linux maps the device special file passed in system calls (say to mount a file system on a block device) to the device's device driver using the major device number and a number of system tables, for example the character device table, chrdevs .

RedHat AS supports three types of hardware device: character, block and network.

    1. Character devices are read and written directly without buffering.

    2. Block devices can only be written to and read from in multiples of the block size, typically 512 or 1024 bytes. Block devices are accessed via the buffer cache and may be randomly accessed, that is to say, any block can be read or written no matter where it is on the device. Block devices can be accessed via their device special file but more commonly they are accessed via the file system. Only a block device can support a mounted file system.

    3. Network devices are accessed via the BSD socket interface and the networking subsytems described in the Networking chapter.

The Raw devices are character devices (major number 162).
The first minor number (i.e. 0) is reserved as a control interface and is usually found at /dev/rawctl.
A sequence of commands listing the raw devices:

    # ls -lR /dev/rawctl
    crw-rw----    1 root     disk     162,   0 Mar  19  2002  /dev/rawctl

    # ls -lR /dev/raw[1-4]
    crw-rw----    1 root     disk     162,   1 Mar  19  2002  /dev/raw1
    crw-rw----    1 root     disk     162,   2 Mar  19  2002  /dev/raw2
    crw-rw----    1 root     disk     162,   3 Mar  19  2002  /dev/raw3
    crw-rw----    1 root     disk     162,   4 Mar  19  2002  /dev/raw4

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Q: 3. What are the benefits of raw devices?

A:A raw device can be bound to an existing block device (e.g. a disk) and be used to perform "raw" IO with that existing block device.
Such "raw" IO bypasses the caching (Linux buffer cache) that is normally associated with block devices and eliminates the file system overheads such as inodes or free lists. Hence a raw device offers a more "direct" route to the physical device and allows an application more control over the timing of IO to that physical device. This makes raw devices suitable for complex applications like Database Management Systems that typically do their own caching.
If there is no I/O bottleneck, raw devices will not help. Note that the overall amount of I/O is not reduced; it is just done more efficiently.

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Q: 4. Are there circumstances when raw devices have to be used?

A:If you are using the Oracle Parallel Server (OPS) or Oracle Real Application Cluster (RAC) without Oracle Cluster File System (OCFS), all data files, control files, and redo log files must be placed on raw partitions so they can be shared between nodes. Also if you use List I/O or Asynchronous I/O, these facilities allow a program to issue multiple write operations without having to wait for the return of the previous write, to take advantage of this data files will need to be on raw devices.

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Q: 5. Can I use the entire raw partition for Oracle?

A:No. You should specify a tablespace slightly smaller in size than the raw partition size, specifically at least two Oracle block sizes smaller.

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Q: 6. How many raw devices I have in RedHat AS by default and how many raw can I have?

A:RedHat AS operating system limits the number of raw devices that Linux can access to 255.
By default on RedHat Advanced Server there are 128 raw devices under /dev/raw:

    # ls -l /dev/raw*
     crw-rw----    1 root     disk     162,   1 Mar  19  2002  /dev/raw1
    (...)
     crw-rw----    1 root     disk     162,   3 Mar  19  2002  /dev/raw128

Linux cannot handle more than a limited number of partitions per drive. So in Linux you have 4 primary partitions (3 of them useable, if you are using logical partitions) and at most 15 partitions altogether on an SCSI disk (63 altogether on an IDE disk).

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Q: 7. How can I create new  raw devices?

A: If it's necessary create others raw devices the following command must be done as root user (see man mknod):

    # mknod -m 660 /dev/raw/rawXXX c 162 XXX
    # chown root:disk /dev/raw/rawXXX
        (where XXX= 128< integer < 256)

    i.e.:
        # mknod -m 660 /dev/raw/raw130 c 162 130
        # chown root:disk /dev/raw/raw130
        # ls -l /dev/raw/raw130
        crw-rw----    1 root     disk     162,   130 Dec  23  18:57  /dev/raw130

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Q:  8. Who should own the raw device?

A:You will need to create the raw devices as root, but the ownership should be changed to the 'oracle' account afterwards. The group must also be changed to the 'dba' group (usually called dba).
  

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Q:  9. How can I use a raw device for Oracle RDBMS?

A:We suppose to have a SCSI disk drivers - 9 Gbytes. The steps are:

    a. Partition the disk driver (/dev/sdb)
    b. Binding raw device with partition on new SCSI disk
    c. Change the ownership to raw device
    d. Create a new Oracle datafile on raw device 

 

[url]http://bbs.chinaunix.net/viewthread.php?tid=59760&extra=page%3D372[/url]

        - Partion the disk driver, fdisk command (see man fdisk):
        1. As user root, type

            # fdisk /dev/sdb

        2. Type 'p' to see the list of existing partitions on your disk drive:

            command (m for help): p
            Disk /dev/sdb: 255 heads, 63 sectors, 1174 cylinders
            Units = cylinders of 16065 * 512 bytes

                Device Boot    Start        End    Block    ID    System

        3.a. In order to create a partition, choose 'n' command and then choose an extended partition with the 'e' option.
             You will need extended partition, because this disk will contains more than 4 partitions.
             Create partition number 1 first, so choose number 1.

            command (m for help): n
            command action
                e    extended
                p    primary partition (1-4)
           e
            Partition Number (1-4): 1
            First cylinder (1-1115, default 1):
            Using default value 1
            Last cylinder or +size or +sizeM or +sizeK (1-1115, default 1115):
            Using default value 1115

        3.b. Now within the extended partition,
             I will have to create 6 logical partition of equal sizes: each should be 257Mb large (256Mb+1Mb for the headers).
             Press 'n' and 'l' and <Enter>;, and write the size of the partition (begin with a +) +257M.
             Repeat these steps 6 times

            command (m for help): n
            command action
                l    logical (5 or over)
                p    primary partition (1-4)
            l
            Partition Number (1-4): 1
            First cylinder (1-1115, default 1):
            Using default value 1
            Last cylinder or +size or +sizeM or +sizeK (1-1115, default 1115): +257M
        (...repeat 5 time...)

            command (m for help): p
            Disk /dev/sdb: 255 heads, 63 sectors, 1174 cylinders
            Units = cylinders of 16065 * 512 bytes

             Device         Boot    Start        End         Block     ID      System
             /dev/sdb1                  1       1115       8956206      5      Extended
             /dev/sdb5                  1         33       265009+     83      Linux
             /dev/sdb6                 34         66       265041      83      Linux
            (...)
             /dev/sdb10               166        198       265041      83      Linux

        3.c. Now press 'w' this will write the partition table to the disk and quit the fdisk programm

        - Binding raw device with partition on new SCSI disk

        A utility called raw (see man raw) can be used to bind a raw device to an existing block device:

            # raw /dev/raw/raw1    /dev/sdb5
            /dev/raw/raw1:  bound to major 8, minor 3
            (...)
            # raw /dev/raw/raw6    /dev/sdb10
            /dev/raw/raw6:  bound to major 8, minor 3

        The last details regarding this is that the assignement of raw device drivers to partitions should be done after each startup.
        For this reason, as user root, edit the /etc/sysconfig/rawdevices and  put the following raw command into it:

            raw /dev/raw/raw1    /dev/sdb5
            raw /dev/raw/raw2    /dev/sdb6
            raw /dev/raw/raw3    /dev/sdb7
            raw /dev/raw/raw4    /dev/sdb8
            raw /dev/raw/raw5    /dev/sdb9
            raw /dev/raw/raw6    /dev/sdb10

        - Change the ownership to raw device

        As root user type:

            # cd /dev/raw
            # chown oracle:dba raw[1-4]

        - Create a new Oracle datafile on raw device

        When using a raw device you need to specify the full pathname in single quotes, and use the REUSE parameter.
        When creating the oracle tablespace on the raw partition a slightly smaller size than the actual partition size needs to be specified.
        This size can be calculated as follows:

                Size of Redo Log   = Raw Partition Size - 1*512 byte block
                Size of Data File    = Raw Partition Size - 2* Oracle Block Size

        e.g. (db_block_size=8192):
            create tablespace tablespace_on_raw datafile '/dev/raw/raw1' size 246784K REUSE,
                                          & '/dev/raw/raw2' size 246784K REUSE,
                                          & '/dev/raw/raw3' size 246784K REUSE,
                                          & '/dev/raw/raw4' size 246784K REUSE,
                                          & '/dev/raw/raw5' size 246784K REUSE,
                                          & '/dev/raw/raw6' size 246784K REUSE;

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Q: 10. Does the Oracle block size have any relevance on a raw device?

A:It is of less importance than for a UNIX file; the size of the Oracle block can be changed, but it must be a multiple of the physical block
    size as it is only possible to seek to physical block boundaries and hence write only in multiples of the physical block size.

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Q: 11. How can I back up my database files if they are on raw devices?

A:You cannot use utilities such as 'tar' or 'cpio', which expect a filesystem to be present.
Usually people move Oracle datafiles from filesystem to raw devices using the 'dd' command. Using dd is the fastest method to accomplish it. However, it is necessary to know how many blocks to skip in the raw device (e.g. on Tru64 Unix you have to skip 64K), so that you do not overwrite information necessary for the Operating System. The information on how many blocks to skip is different on the different platforms. Using RMAN there's no necessity to know such platform specific information. With the RMAN copy command datafiles can be
copied from filesystem files to raw devices.

    # dd if=/dev/raw/raw1 of=/u01/oradata/test_ts.dbf'  bs=16K
    (Keep the Block size to multiple of the Oracle Block Size)

    See the UNIX man page on dd for further details.

You can use RMAN.
From filesystem to raw device:

    RMAN>; run {
    2>; allocate channel c1 type disk;
    3>; copy datafile '/u01/oradata/test_ts.dbf' to '/dev/raw/raw1';
    4>; }

From raw device to filesystem:

    RMAN>; run {
    2>; allocate channel c1 type disk;
    3>; copy datafile '/dev/raw/raw1' to '/u01/oradata/test_ts.dbf';
    4>; }

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Q: 12. Providing I am not using Parallel Server or Real Application Cluster, can I use a mixture of raw?

A:Yes. The drawback is that this makes your backup strategy more complicated.

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Q: 13. Should I store my redo log files on raw partitions?

A:Redo logs are particularly suitable candidates for being located on raw partitions, as they are write-intensive and in addition are written to
    sequentially. If OPS or RAC is being used, redo logs must be stored on raw partitions.

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Q: 14. Can I use raw partitions for archive logs?

A:No. Archive logs must be stored on a partition with a UNIX filesystem.

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Q:  15. Can I have more than one data file on a raw partition?

A:No. This means you should be careful when setting up the raw partition. Too small a size will necessitate reorganisation when you
    run out of space, whereas too large a size will waste any space the file does not use.

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Q: 16. Should my raw partitions be on the same disk device?

A:This is inadvisable, as there is likely to be contention. You should place raw devices on different disks, which should also be on different
    controllers.
  

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Q: 17. Do I need to make my raw partitions all the same size?
A:This is not essential, but it provides flexibility in the event of having to change the database configuration.
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Q: 18. Do I need to change any UNIX kernel parameters if I decide to use raw devices?
A:No

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Q: 19. What other UNIX-level changes could help to improve I/O performance?

A:RAID and disk mirroring can be beneficial, depending on the application characteristics, especially whether it is read or write-intensive, or a
    mixture.

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Q: 20. How can I gain further performance benefits, after considering all of the above?

A:You will need to buy more disk drives and controllers for your system, to spread the I/O load between devices.