What is different Linux File Systems: Ext2 vs Ext3 & Ext4

ext2, ext3 and ext4 are all filesystems created for Linux. 

• High level difference between these filesystems.
• How to create these filesystems.
• How to convert from one filesystem type to another.

Ext2

• Ext2 stands for second extended file system.
• It was introduced in 1993. Developed by Rémy Card.
• This was developed to overcome the limitation of the original ext file system.
• Ext2 does not have journaling feature.
• On flash drives, usb drives, ext2 is recommended, as it doesn’t need to do the over head of journaling.
• Maximum individual file size can be from 16 GB to 2 TB
• Overall ext2 file system size can be from 2 TB to 32 TB

Ext3

• Ext3 stands for third extended file system.
• It was introduced in 2001. Developed by Stephen Tweedie.
• Starting from Linux Kernel 2.4.15 ext3 was available.
• The main benefit of ext3 is that it allows journaling.
• Journaling has a dedicated area in the file system, where all the changes are tracked. When the system crashes, the possibility of file system corruption is less because of journaling.
• Maximum individual file size can be from 16 GB to 2 TB
• Overall ext3 file system size can be from 2 TB to 32 TB
• There are three types of journaling available in ext3 file system.
  • Journal – Metadata and content are saved in the journal.
  • Ordered – Only metadata is saved in the journal. Metadata are journaled only after writing the content to disk. This is the default.
  • Writeback – Only metadata is saved in the journal. Metadata might be journaled either before or after the content is written to the disk.
• You can convert a ext2 file system to ext3 file system directly (without backup/restore).

Ext4

• Ext4 stands for fourth extended file system.
• It was introduced in 2008.
• Starting from Linux Kernel 2.6.19 ext4 was available.
• Supports huge individual file size and overall file system size.
• Maximum individual file size can be from 16 GB to 16 TB
• Overall maximum ext4 file system size is 1 EB (exabyte). 1 EB = 1024 PB (petabyte). 1 PB = 1024 TB (terabyte).
• Directory can contain a maximum of 64,000 subdirectories (as opposed to 32,000 in ext3)
• You can also mount an existing ext3 fs as ext4 fs (without having to upgrade it).
• Several other new features are introduced in ext4: multiblock allocation, delayed allocation, journal checksum. fast fsck, etc. All you need to know is that these new features have improved the performance and reliability of the filesystem when compared to ext3.
• In ext4, you also have the option of turning the journaling feature “off”.

Use the method we discussed earlier to identify whether you have ext2 or ext3 or ext4 file system.

Warning: Don’t execute any of the commands given below, if you don’t know what you are doing. You will lose your data!

Creating an ext2, or ext3, or ext4 filesystem

Once you’ve partitioned your hard disk using fdisk command, use mke2fs to create either ext2, ext3, or ext4 file system.

Create an ext2 file system:

mke2fs /dev/sda1

Create an ext3 file system:

mkfs.ext3 /dev/sda1

(or)

mke2fs –j /dev/sda1

Create an ext4 file system:

mkfs.ext4 /dev/sda1

(or)

mke2fs -t ext4 /dev/sda1

Converting ext2 to ext3

For example, if you are upgrading /dev/sda2 that is mounted as /home, from ext2 to ext3, do the following.

umount /dev/sda2

tune2fs -j /dev/sda2

mount /dev/sda2 /home

Note: You really don’t need to umount and mount it, as ext2 to ext3 conversion can happen on a live file system. But, I feel better doing the conversion offline.

Converting ext3 to ext4

If you are upgrading /dev/sda2 that is mounted as /home, from ext3 to ext4, do the following.

umount /dev/sda2

tune2fs -O extents,uninit_bg,dir_index /dev/sda2

e2fsck -pf /dev/sda2

mount /dev/sda2 /home

Again, try all of the above commands only on a test system, where you can afford to lose all your data.

What is LUN

 A logical unit number (LUN) is a unique identifier used to designate individual or collections ofhard disk devices for address by a protocol associated with a SCSI, iSCSI, Fibre Channel (FC) or similar interface. LUNs are central to the management of block storage arrays shared over a storage area network (SAN). 

The term LUN dates back to the early days of SCSI when each device was identified by a logical number, up to eight in those days. Now servers with a dozen or more LUNs are common and it's getting less common for them to be connected to a conventional internal SCSI disk array. However, the basic element of storage for the server is still referred to as the LUN.

Each LUN identifies a specific logical unit, which may be a part of a hard disk drive, an entire hard disk or several hard disks in a storage device. So a LUN could reference an entire RAIDset, a single disk or partition, or multiple hard disks or partitions. In any case, the logical unit is treated as if it is a single device and is identified by the LUN.

Here's how LUNs work with SCSI:
A SCSI (Small System Computer Interface) is a parallel interface that can have up to eight devices all attached through a single cable; the cable and the host (computer) adapter make up the SCSI bus. The bus allows the interchange of information between devices independently of the host. In the SCSI program, each device is assigned a unique number, which is either a number between 0 and 7 for an 8-bit (narrow) bus, or between 8 and 16 for a 16-bit (wide) bus. The devices that request input/output (I/O) operations are initiators and the devices that perform these operations are targets. Each target has the capacity to connect up to eight additional devices through its own controller; these devices are the logical units, each of which is assigned a unique number for identification to the SCSI controller for command processing.

In LUN zoning, SAN fabric is configured to match LUNs to the proper servers. As a rule, end devices such as hosts can only see and access storage within their zone. Limiting access in this way improves security and allows bandwidth allocation through assigning particular ports to a zone.

LUN masking is a further constraint added to zoning, subdividing access to the port so that only LUNs authorized to access a specific server can access the corresponding port. Then, even if several LUNs are accessed through the same port, the server masks can be set to limit each server's access to the appropriate LUNs. LUN masking is typically conducted at the host bus adapter (HBA) or switch level.

How to install modules In Liunx

Ans. odprobe utility is used to add loadable modules to the Linux kernel. You can also view and remove modules using modprobe command.

Linux maintains /lib/modules/$(uname-r) directory for modules and its configuration files (except /etc/modprobe.conf and /etc/modprobe.d).

In Linux kernel 2.6, the .ko modules are used instead of .o files since that has additional information that the kernel uses to load the modules. The example in this article are done with using modprobe on Ubuntu.

1. List Available Kernel Modules

modprobe -l will display all available modules as shown below.

$ modprobe -l | less

kernel/arch/x86/kernel/cpu/mcheck/mce-inject.ko

kernel/arch/x86/kernel/cpu/cpufreq/e_powersaver.ko

kernel/arch/x86/kernel/cpu/cpufreq/p4-clockmod.ko

kernel/arch/x86/kernel/msr.ko

kernel/arch/x86/kernel/cpuid.ko

kernel/arch/x86/kernel/apm.ko

kernel/arch/x86/kernel/scx200.ko

kernel/arch/x86/kernel/microcode.ko

kernel/arch/x86/crypto/aes-i586.ko

kernel/arch/x86/crypto/twofish-i586.ko

2. List Currently Loaded Modules

While the above modprobe command shows all available modules, lsmod command will display all modules that are currently loaded in the Linux kernel.

$ lsmod | less

soundcore          7264   1 snd

ppdev                6688   0

snd_page_alloc   9156    1 snd_pcm

psmouse            56180  0

lp                     8964    0

3. Install New modules into Linux Kernel

In order to insert a new module into the kernel, execute the modprobe command with the module name.

Following example loads vmhgfs module to Linux kernel on Ubuntu.

$ sudo modprobe vmhgfs

Once a module is loaded, verify it using lsmod command as shown below.

$ lsmod | grep vmhgfs

vmhgfs                 50772  0

The module files are with .ko extension. If you like to know the full file location of a specific Linux kernel module, use modprobe command and do a grep of the module name as shown below.

$ modprobe | grep vmhgfs

misc/vmhgfs.ko

$ cd /lib/modules/2.6.31-14-generic/misc

$ ls vmhgfs*

vmhgfs.ko

Note: You can also use insmod for installing new modules into the Linux kernel.

4. Load New Modules with the Different Name to Avoid Conflicts

Consider, in some cases you are supposed to load a new module but with the same module name another module got already loaded for different purposes.

If for some strange reasons, the module name you are trying to load into the kernel is getting used (with the same name) by a different module, then you can load the new module using a different name.

To load a module with a different name, use the modprobe option -o as shown below.

$ sudo modprobe vmhgfs -o vm_hgfs

$ lsmod  | grep vm_hgfs

vm_hgfs                   50772  0

5. Remove the Currently Loaded Module

If you’ve loaded a module to Linux kernel for some testing purpose, you might want to unload (remove) it from the kernel.

Use modprobe -r option to unload a module from the kernel as shown below.

modprobe -r vmhgfs