Commands Reference, Volume 3

mkramdisk Command

Purpose

Creates a RAM disk using a portion of RAM that is accessed through normal reads and writes.

Syntax

mkramdisk [ -u ] size[ M | G ]

Description

The mkramdisk command is shipped as part of bos.rte.filesystems, which allows the user to create a RAM disk. Upon successful execution of the mkramdisk command, a new RAM disk is created, a new entry added to /dev, the name of the new RAM disk is written to standard output, and the command exits with a value of 0. If the creation of the RAM disk fails, the command prints an internalized error message, and the command will exit with a nonzero value.

The size can be specified in terms of MB or GB. By default, it is in 512 byte blocks. Suffix M will be used to specify size in megabytes and G to specify size in gigabytes.

The names of the RAM disks are in the form of /dev/rramdiskx where x is the logical RAM disk number (0 through 63).

The mkramdisk command also creates block special device entries (for example, /dev/ramdisk5) although use of the block device interface is discouraged because it adds overhead. The device special files in /dev are owned by root with a mode of 600. However, the mode, owner, and group ID can be changed using normal system commands.

Up to 64 RAM disks can be created.

Note

The size of a RAM disk cannot be changed after it is created.

The mkramdisk command is responsible for generating a major number, loading the ram disk kernel extension, configuring the kernel extension, creating a ram disk, and creating the device special files in /dev. Once the device special files are created, they can be used just like any other device special files through normal open, read, write, and close system calls.

RAM disks can be removed by using the rmramdisk command. RAM disks are also removed when the machine is rebooted.

To set up a RAM disk that is approximately 20 MB in size and create a file system on that RAM disk, enter the following:

mkramdisk 40000
ls -l /dev | grep ram
mkfs -V jfs /dev/ramdiskx
mkdir /ramdisk0
mount -V jfs -o nointegrity /dev/ramdiskx /ramdiskx

where x is the logical RAM disk number.

By default, RAM disk pages are pinned. Use the -u flag to create RAM disk pages that are not pinned.

Flags

-u	Specifies that the ram disk that is created will not be pinned. By default, the ram disk will be pinned.

Parameters

size	Indicates the amount of RAM (in 512 byte increments) to use for the new RAM disk. For example, typing: mkramdisk 1 creates a RAM disk that uses 512 bytes of RAM. To create a RAM disk that uses approximately 20 MB of RAM, type: mkramdisk 40000

Exit Status

The following exit values are returned:

0	Successful completion.
>0	An error occurred.

Examples

To create a new ram disk using a default 512-byte block size, and the size is 500 MBs (1048576 * 512), enter:
```
     $ mkramdisk 1048576
       /dev/rramdisk0
```
The /dev/rramdisk0 ramdisk is created.
To create a new ramdisk with a size of 500 Megabytes, enter:
```
     $ mkramdisk 500M
       /dev/rramdisk0
```
The /dev/rramdisk0 ramdisk is created. Note that the ramdisk has the same size as example 1 above.
To create a new ram disk with a 2-Gigabyte size, enter:
```
     $ mkramdisk 2G
       /dev/rramdisk0
```

Files

/usr/sbin/mkramdisk

Contains the mkramdisk command.

Related Information

The rmramdisk command.