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ramdisk(HW)

ramdisk -- memory block device

Description

The ramdisk device driver provides a block interface to memory. A ramdisk can be used like any other block device, including making it into a filesystem using mkfs(ADM). Up to eight ramdisks may be created.

The characteristics of a ramdisk file are determined by its minor device number. The bits in the minor device number encode its size, longevity, and which of the eight possible ramdisks it is.

The three low-order bits of the minor device number determine the ramdisk number.

The next four bits of the minor device number determine the size of the ramdisk. There are 16 possible ramdisk sizes from 16KB to 512MB.

The possible settings of the size bits and the corresponding ramdisk sizes are shown in the following table.

Size bit settings

Size bits Value Ramdisk size
0 0 0 0 0 16KB
0 0 0 1 8 32KB
0 0 1 0 16 64KB
0 0 1 1 24 128KB
0 1 0 0 32 256KB
0 1 0 1 40 512KB
0 1 1 0 48 1MB
0 1 1 1 56 2MB
1 0 0 0 64 4MB
1 0 0 1 72 8MB
1 0 1 0 80 16MB
1 0 1 1 88 32MB
1 1 0 0 96 64MB
1 1 0 1 104 128MB
1 1 1 0 112 256MB
1 1 1 1 120 512MB 

 +---------------------+---------+-----------------------+
 |     Size bits       |  Value  |  Ramdisk size         |
 +---+-----+-----+-----++--------++----------------------+
 |0  |  0  |  0  |  0  ||     0  ||                 16KB |
 +---+-----+-----+-----++--------++----------------------+
 |0  |  0  |  0  |  1  ||     8  ||                 32KB |
 +---+-----+-----+-----++--------++----------------------+
 |0  |  0  |  1  |  0  ||    16  ||                 64KB |
 +---+-----+-----+-----++--------++----------------------+
 |0  |  0  |  1  |  1  ||    24  ||                128KB |
 +---+-----+-----+-----++--------++----------------------+
 |0  |  1  |  0  |  0  ||    32  ||                256KB |
 +---+-----+-----+-----++--------++----------------------+
 |0  |  1  |  0  |  1  ||    40  ||                512KB |
 +---+-----+-----+-----++--------++----------------------+
 |0  |  1  |  1  |  0  ||    48  ||                  1MB |
 +---+-----+-----+-----++--------++----------------------+
 |0  |  1  |  1  |  1  ||    56  ||                  2MB |
 +---+-----+-----+-----++--------++----------------------+
 |1  |  0  |  0  |  0  ||    64  ||                  4MB |
 +---+-----+-----+-----++--------++----------------------+
 |1  |  0  |  0  |  1  ||    72  ||                  8MB |
 +---+-----+-----+-----++--------++----------------------+
 |1  |  0  |  1  |  0  ||    80  ||                 16MB |
 +---+-----+-----+-----++--------++----------------------+
 |1  |  0  |  1  |  1  ||    88  ||                 32MB |
 +---+-----+-----+-----++--------++----------------------+
 |1  |  1  |  0  |  0  ||    96  ||                 64MB |
 +---+-----+-----+-----++--------++----------------------+
 |1  |  1  |  0  |  1  ||   104  ||                128MB |
 +---+-----+-----+-----++--------++----------------------+
 |1  |  1  |  1  |  0  ||   112  ||                256MB |
 +---+-----+-----+-----++--------++----------------------+
 |1  |  1  |  1  |  1  ||   120  ||                512MB |
 +---+-----+-----+-----++--------++----------------------+
The high-order bit is a longevity indicator. If set (value 128), memory is permanently allocated to that ramdisk, and can be deallocated only by rebooting the system. Permanent ramdisks can only be allocated by the superuser. However, once a permanent ramdisk is allocated (by opening it), it can be read and written by anyone having the appropriate permissions on the ramdisk inode.

If the high-order bit is clear (value 0), the ramdisk is temporary and will be deallocated when no processes have it open. An unmounted ramdisk will be deallocated immediately when it is closed. To create an easily removable, but semi-permanent ramdisk, use a separate process to keep the device open for as long as necessary.

The following table shows examples of how the minor device number is constructed:

Examples of minor device number construction

Description Longevity Size RAM Minor
disk number device
number
16KB (#1)  (Temporary) 0 0 0 0 0 0 0 1 1
16KB (#1)  (Permanent) 1 0 0 0 0 0 0 1 129
64KB (#0)  (Temporary) 0 0 0 1 0 0 0 0 16
512KB (#7) (Permanent) 1 0 1 0 1 1 1 1 175 

 +-----------------------+-----------+---------------+-------------+--------+
 |Description            | Longevity |     Size      |     RAM     | Minor  |
 +-----------------------+-----------+---------------+-------------+--------+
 |                       |           |               | disk number | device |
 +-----------------------+-----------+---------------+-------------+--------+
 |                       |           |               |             | number |
 +-----------------------++----------++--+---+---+---++---+----+---++-------+
 |16KB (#1)              ||        0 ||0 | 0 | 0 | 0 || 0 | 0  | 1 ||     1 |
 | (Temporary)           ||          ||  |   |   |   ||   |    |   ||       |
 +-----------------------++----------++--+---+---+---++---+----+---++-------+
 |16KB (#1)              ||        1 ||0 | 0 | 0 | 0 || 0 | 0  | 1 ||   129 |
 | (Permanent)           ||          ||  |   |   |   ||   |    |   ||       |
 +-----------------------++----------++--+---+---+---++---+----+---++-------+
 |64KB (#0)              ||        0 ||0 | 0 | 1 | 0 || 0 | 0  | 0 ||    16 |
 | (Temporary)           ||          ||  |   |   |   ||   |    |   ||       |
 +-----------------------++----------++--+---+---+---++---+----+---++-------+
 |512KB (#7) (Permanent) ||        1 ||0 | 1 | 0 | 1 || 1 | 1  | 1 ||   175 |
 +-----------------------++----------++--+---+---+---++---+----+---++-------+
Only one special device file (/dev/ram00 corresponding to a temporary 16KB ramdisk) is created when the system is installed. All ramdisks created by the system administrator share the same major device number as /dev/ram00.

The major device number to be used when creating ramdisks can be found by entering:

ls -l /dev/ram00 | awk '{print $5}' | sed 's/,//'.

To create a ramdisk, follow these steps:

  1. Create the device node.

    First create the device that the ramdisk will reside on. Use the mknod command:

    mknod device_name [b|c] 31 minor_device_number

    ``b'' is for blocked devices and is the one you will use. ``31'' is the major number for this type of device. The minor number is the sum of the values of three attributes representing longevity, size, and ramdisk number.

    For example, to create a 64KB permanent ramdisk, the minor numbers 144 to 151 represent ramdisk numbers 0 to 7. If the disk number was 1, the mknod command would be:

    mknod /dev/ram145 b 31 145

  2. Make a filesystem.

    Use mkfs to create a filesystem on the ramdisk (see step 3 for details of how to create a filesystem on a temporary ramdisk):

    mkfs device_name filesystem_size_in_physical_blocks

    The physical block size is 512 bytes so two blocks need to be specified for each 1KB of filesystem. In this example, the command to create a 64KB filesystem would be:

    mkfs /dev/ram145 128

  3. Mount the filesystem.

    Use mount(ADM) to mount the selected device on the specified mountpoint:

    mount device_name mount_point

    In order to mount the example 64KB ramdisk on /mnt, the command would be:

    mount /dev/ram145 /mnt

    To make a filesystem on a temporary ramdisk, the device file must be held open between the mkfs and the mount operations. Otherwise, the ramdisk is allocated at the start of the mkfs command, and deallocated at its end, before it can be mounted. Once the ramdisk is mounted, it is open until it is unmounted.

The following shell fragment shows one method of creating and mounting a filesystem on a non-permanent 512KB ramdisk: 
( 	/etc/mkfs /dev/ram40 1024
	/etc/mount /dev/ram40 /mnt
) < /dev/ram40
The procedure is executed in a sub-shell taking its standard input from /dev/ram40 in order to keep /dev/ram40 open between its creation and the time at which it is mounted.

Limitations

A maximum of eight ramdisks are available.

Two different sized ramdisks may not share the same ramdisk number.

Ramdisks must occupy contiguous memory. If free memory is fragmented, opening a ramdisk may fail even though there is enough total memory available. Ideally, all ramdisks should be allocated at system startup. This helps prevent the ramdisks themselves from fragmenting memory.

Ramdisks are geared towards use in specialized applications. In many cases, you will notice a decrease in system performance when ramdisks are used, because UNIX can generally put the memory to better use elsewhere.

Files

/dev/ram00

See also

mkfs(ADM), mknod(C), mount(ADM)

Standards conformance

ramdisk is not part of any currently supported standard; it is an extension of AT&T System V provided by The Santa Cruz Operation, Inc.
© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003