what is the form of filesystem like ext3, ext2, ext

Question

i've read filesystem in wikipedia, linfo.org and a question on super user about "is filesystem a part of operating system" and i doubt my understanding.

Wikipedia says: "ext is filesystem that is commonly used by linux kernel".
SU's answer says: "the OS contains a driver that allows it to work with filesystem"

now what is the form of the ext itself? is it a driver, used by linux to organize data on disk?

Answer 1

Unfortunately, what "filesystem" exactly means depends on the context.

Most commonly, "filesystem" describes the on-disk format of volumes/partitions. In that sense, APFS, ext, FAT32, and XFS are some of different filesystems. For example, you may hear people say something like "APFS supports alternate data streams, XFS doesn't".

Many times the term "filesystem" is used to describe the ecosystem: the on-disk format of volumes/partitions, plus the OS drivers that read and write these formats. For example, you may hear people say something like "ext2 is not crash-safe, upgrade your partitions to ext3" (see below).

Sometimes, the term "filesystem" is used to describe the driver that reads and writes the on-disk format. For example, you may hear people say something like "ZFS doesn't work on windows, but NFS works everywhere".

NFS, in particular, has a driver, but doesn't have an on-disk format, because it just asks a remote server to store everything on its own filesystem, whichever that is.

The distinction between on-disk format and the driver which reads and writes it is particularly confusing for ext2/3/4. The ext family of filesystem drivers shares a common on-disk format - if you dig up an ext* partition, nowhere on the partition will it say "my version is ext3". Instead, the partition will have a list of features.

What does have different versions is the driver - there are drivers for ext2, ext3, and ext4, and each version adds support for new features. So, you can create a partition using the ext3 driver, and the partition will use features supported by the ext3 driver - both the ext3 and ext4 drivers can read it. Then you upgrade it using the ext4 driver in order to use a new on-disk feature (like high-resolution timestamps), but then you have to read it using the ext4 driver - the ext3 driver can no longer read it because it doesn't support the new feature.

Occasionally you may also see "filesystem" referring to a specific volume/partition that has files and directories. For example, you may hear people say something like "My filesystem is corrupted".