Linux with Operating System Concepts

ramdisks
. The ramdisk is an older idea whereby RAM is used to simulate disk. That is, 
items from disk are loaded into RAM so that access to those items can be performed far 
more efficiently. As the ramdisks are limited in size compared to the file space available, 
you have to load the ramdisks wisely. Commonly, files that are receiving multiple disk 
accesses are loaded into available ramdisks until they are no longer needed (or until newer 
file accesses arise).
The ramdisk dates back to around 1980. Early personal computers had a limited address 
space for memory, in part caused by the limited number of bits that could be stored in 
registers or carried across the address bus. However, some PC owners were willing to buy 
additional memory. Early PC operating systems would load select files into ramdisk in 
anticipation of being used in the near future. The downside to this technology was that 
RAM memory was still very expensive and so a ramdisk might not be practical for many 
home computer users.
Linux systems offer ramdisks under two formats. First, there are specific ramdisk 
devices stored under /dev/ram. Second is the tmpfs partition that provides shared memory 
in the form of a ramdisk.
Finally, there are a number of tty devices. These are available terminal windows. You 
might find 64 tty’s and 4 ttyS’s, the latter of which are serial consoles.
The /proc directory contains information generated by the Linux kernel about the run-
ning processes. Each running process is indicated by its own subdirectory whose name 
matches the PID for that process. An example of this directory is shown in Figure 10.9. 
The directories with names such as 1, 10, 1041, through 2225 are all of processes. There are 
FIGURE 10.9 
Contents of the /proc directory.

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Linux with Operating System Concepts
other directories containing kernel generation information about the system itself such as 
acpi, bus, driver, fs, and irq. The remainder of the content of this directory are files about 
system processes.
The files stored both within /proc and in the subdirectories are not true files but instead 
information about the processes. You will find that all of these have a size of 0. Some of 
the files seen in the /proc directory also occur within each subdirectory such as cgroups, 
cmdline, schedstat, and stat. Others are unique for running processes (e.g., environ, maps, 
mounts, pagemap, and status) and others are unique for the kernel.
Here we examine some of the contents for the processes.
• cmdline—the command line instruction that launched this process (if launched from 
the command line, empty otherwise)
• environ—this process’s environment variables (if any)
• fd—file descriptors (open files)
• io—this process’ I/O utilization
• limits—the limits under which this process was established (e.g., maximum amount 
of CPU time, maximum file size, and maximum memory utilization)
• mounts—mount information
• cwd—a link to the current working directory of the process
• exe—a link to the process’ executable file
• root—a link to the root directory of the process
For the system files, the cmdline file stores the kernel’s startup instruction, including 
all parameters supplied. Other files stored in /proc include information on CPU usage, 
memory usage, average CPU load, file system usage, mounted partitions and their usage, 
and virtual memory usage.
Whereas /proc stores information about running processes, /sys stores information 
about devices and device drivers along with device configurations. The /sys directory’s file 
system is called sysfs. In /sys, you will find subdirectories for block devices, the bus, firm-
ware, file systems, and power management. You will also find subdirectories storing crash/
core information about the kernel. A quick look at this directory’s long listing will show 
you that, like /proc, the “files” here have a size of 0.
10.5.3 The /etc Directory
Although part of the root partition, we consider the /etc directory separately because it 
is a directory that you, as a system administrator, will be involved with most often. This 
directory stores