Linux File Systems: Ext2 vs Ext3 vs Ext4 vs XFS

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

This article explains the following:

• High a level of difference between these filesystems.

• How to create these filesystems.

• How to convert from one filesystem type to another.

Ext2

• Ext2 stands for the 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”.

XFS File System

• The XFS file system is an extension of the extent file system.

• The XFS is a high-performance 64-bit journaling file system.

• The support of the XFS was merged into Linux kernel in around 2002 and In 2009 Red Hat Enterprise Linux version 5.4 usage of the XFS file system.

• XFS supports maximum file system size of 8 exbibytes for the 64-bit file system.

• There is some comparison of XFS file system is XFS file system can’t be shrunk and poor performance with deletions of the large numbers of files.

• Now, the RHEL 7.0 uses XFS as the default filesystem.

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.

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What is a Journaling Filesystem?

A journaling filesystem keeps a journal or log of the changes that are being made to the filesystem during disk writing that can be used to rapidly reconstruct corruptions that may occur due to events such a system crash or power outage. The level of journaling performed by the file system can be configured to provide a number of levels of logging depending on your needs and performance requirements.

What are the Advantages of a Journaling Filesystem?

There are a number of advantages to using a journaling files system.

Both the size and volume of data stored on disk drives has grown exponentially over the years. The probelm with a non-journaled file system is that following a crash the fsck (filesystem consistency check) utility has to be run. fsck will scan the entire filesystem validating all entries and making sure that blocks are allocated and referenced correctly. If it finds a corrupt entry it will attempt to fix the problem. The issues here are two-fold. Firstly, the fsck utility will not always be able to repair damage and you will end up with data in the lost+found directory. This is data that was being used by an application but the system no longer knows where they were reference from. The other problem is the issue of time. It can take a very long time to complete the fsck process on a large file system leading to unacceptable down time.

A journaled file system records information in a log area on a disk (the journal and log do not need to be on the same device) during each write. This is a essentially an "intent to commit" data to the filesystem. The amount of information logged is configurable and ranges from not logging anything, to logging what is known as the "metadata" (i.e ownership, date stamp information etc), to logging the "metadata" and the data blocks that are to be written to the file. Once the log is updated the system then writes the actual data to the appropriate areas of the filesystem and marks an entry in the log to say the data is committed.

After a crash the filesystem can very quickly be brought back on-line using the journal log reducing what could take minutes using fsck to seconds with the added advantage that there is considerably less chance of data loss or corruption.

What is a Journal Checkpoint?

When a file is accessed on the filesystem, the last snapshot of that file is read from the disk into memory. The journal log is then consulted to see if any uncommitted changes have been made to the file since the data was last written to the file (essentially looking for an "intention to commit" in the log entry as described above). At particular points the filesystem will update file data on the disk from the uncommited log entries and trim those entries from the log. Committing operations from the log and synchronizing the log and its associated filesystem is called a checkpoint.

What are the disadvantages of a Journaled Filesystem?

Nothing in life is is free and ext3 and journaled filesystems are no exception to the rule. The biggest draw back of journaling is in the area of performance simply because more disk writes are required to store information in the log. In practice, however, unless you are running system where disk performance is absolutely critical the performance difference will be negligable.

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Common Commands for ext3 and ext4 Compared to XFS