Memory file systems and virtual file systems
These file systems do not exist on disk in the
same way that traditional file systems do. They either exist entirely in system
memory, or they are virtual because they are an interface to system devices.
Some of them are :
CRAMFS
a. cramfs is designed to cram a file
system onto a small ROM, so it is small, simple, and able to compress things
well. The largest file size is 16MB, and the largest file system size is 256MB.
b. Since cramfs is so compressed, it
isn’t instantly updateable. The mkcramfs tool needs to be run to create or
update a cramfs disk image.
c. The image is created by compressing
files one page at a time, so this enables random page access.
d. The metadata is not compressed, but it
has been optimized to take up much less space than other file systems.
e. For example, only the low 8 bits of
the gid are stored. This saves space but also presents a potential security
issue.
TMPFS
a. tmpfs is structured around the idea
that whatever is put in the /tmp file system is accessed again shortly.
b. tmpfs exists solely in memory, so what
you put in temp doesn’t persist between reboots.
c. Creating /tmp as an in-memory file
system is a performance boost. Creating /tmp as an in-memory file system is
done in Solaris since the overhead of Solaris is very large.
d. Creating /tmp as an in-memory file
system hasn’t been done before in Linux because the ext2 file system has pretty
good performance already.
e. But for those who feel that they need
the performance gains of storing /tmp in memory, this option is now available
in Linux.
a. ramfs is basically cramfs without the
compression.
a. /dev/pts is a lightweight version of
devfs. Instead of having all the device files supported in the virtual file
system, it provides support for only virtual pseudo terminal device files.
/dev/pts was implemented before devfs.
DEVFS
a. The Device File System (devfs) is
another way to access “real” character and block special devices on your root
file system.
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