RAMDISK(HW) UNIX System V
Name
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 file systems using mkfs(ADM).
There are eight ramdisks available.
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 which of the eight ramdisks is being accessed.
The next four bits of the minor device number determine the
size of the ramdisk. The size of a ramdisk must be a power
of 2, and must be at least 16K. Since 4 bits are available,
there are 16 possible sizes, starting at 16K and doubling
every time the size indicator is incremented, to produce
possible sizes of 16K, 32K, 64K, and up.
The high-order bit is a longevity indicator. If set, 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 clear, the ramdisk is deallocated when no processes have
it open. To create an easily removable, but semi-permanent
ramdisk, use a separate process to keep the device open for
as long as necessary.
Since a complete set of ramdisks (8) would consume 256
inodes, only one example 16K ramdisk (/dev/ram00) is created
when the system is installed. The system administrator can
check this existing file to determine the major device
number for any other required ramdisks. All ramdisks will
use the same major device number.
The following table shows how the minor device number is
constructed:
_______________________________________________________________
| Example Minor Device Number Construction |
|______________________________________________________________|
| Description Longe- Size (see Ram Minor |
| vity next table) Disk No. Device|
| Number|
|________________|________|___|___|___|___|___|___|___|________|
| 16K (#1) | 0 | 0| 0| 0| 0| 0| 0| 1| 1 |
| (Temporary) | | | | | | | | | |
| 16K (#1) | 1 | 0| 0| 0| 0| 0| 0| 1| 129 |
| (Permanent) | | | | | | | | | |
| 64K (#0) | 0 | 0| 0| 1| 0| 0| 0| 0| 16 |
| (Temporary) | | | | | | | | | |
| 512K (#7) | 1 | 0| 1| 0| 1| 1| 1| 1| 175 |
| (Permanent) | | | | | | | | | |
|________________|________|___|___|___|___|___|___|___|________|
The contents of the size field and the corresponding ramdisk
size is shown in the next table.
______________________________
| Size Bits | Ramdisk Size|
|__|___|___|___|______________|
| 0| 0| 0| 0| 16K |
|__|___|___|___|______________|
| 0| 0| 0| 1| 32K |
|__|___|___|___|______________|
| 0| 0| 1| 0| 64K |
|__|___|___|___|______________|
| 0| 0| 1| 1| 128K |
|__|___|___|___|______________|
| 0| 1| 0| 0| 256K |
|__|___|___|___|______________|
| 0| 1| 0| 1| 512K |
|__|___|___|___|______________|
| 0| 1| 1| 0| 1M |
|__|___|___|___|______________|
| 0| 1| 1| 1| 2M |
|__|___|___|___|______________|
| 1| 0| 0| 0| 4M |
|__|___|___|___|______________|
| 1| 0| 0| 1| 8M |
|__|___|___|___|______________|
| 1| 0| 1| 0| 16M |
|__|___|___|___|______________|
| 1| 0| 1| 1| 32M |
|__|___|___|___|______________|
| 1| 1| 0| 0| 64M |
|__|___|___|___|______________|
| 1| 1| 0| 1| 128M |
|__|___|___|___|______________|
| 1| 1| 1| 0| 256M |
|__|___|___|___|______________|
| 1| 1| 1| 1| 512M |
|__|___|___|___|______________|
To create a ramdisk, follow these steps:
1. Create the device node.
You must first create the device that the ramdisk will
reside on. It has the form:
mknod device_name b_or_c major_device_number minor_device_number
where b_or_c ``b'' or ``c''. `'b'' is for blocked devices
and is the one you will use. The major number will always
be 31. The minor number is derived from the table above.
The minor number is the sum of the three attribute columns.
Longevity:
permanent = 128 non-permanent = 0
Size:
16K = 0 128K = 24 1 Meg = 48 8 Meg = 72
32K = 8 256K = 32 2 Meg = 56 16 Meg = 80
64K = 16 512K = 40 4 Meg = 64 32 Meg = 88
Ram Disk number: 0 through 7 Note: There are only 8 devices
available. Two different size devices may not share the
same number.
For example, to create a 64K permanent ramdisk, the minor
number could vary from 144 to 151. If the disk number was 1
the mknod command would be:
mknod /dev/ram64 b 31 145
2. Make a file system.
This creates a file system on the the ramdisk. In this
example mkfs has the form:
mkfs device_name size_of_file_in_Bsize_blocks
In this example, the command to create a 64K file system
would be:
mkfs /dev/ram64 64
3. Mount the filesystem.
This mounts the selected device on the specified mount
point. It has the form:
mount device_name mount_point
In order to mount the example 64K ramdisk on /mnt the
command would be:
mount /dev/ram64 /mnt
To make a file system on a non-permanent ramdisk, the device
file must be held open between the mkfs and the mount(ADM)
operations. Otherwise, the ramdisk is allocated at the
start of the mkfs command, and deallocated at its end. Once
the ramdisk is mounted, it is open until it is unmounted.
The following shell fragment shows one way to use mkfs on a
non-permanent 512K ramdisk, then mount it:
( /etc/mkfs /dev/ram40 512
/etc/mount /dev/ram40 /mnt
) < /dev/ram40
Notes
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), mount(ADM), mknod(C)
Value Added
ramdisk is an extension of AT&T System V provided by the
Santa Cruz Operation.
(printed 2/15/90) RAMDISK(HW)