Ext4 in Fedora 11

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Frequently Asked Questions

What's the benefit from Ext4?

Improved performance (especially for filesystem checks and file deletions), higher storage capacity and more scalability.

http://kernelnewbies.org/Ext4

How long has Fedora supported Ext4?

Fedora 9 originally included experimental support for Ext4. This was a Anaconda boot time option in Fedora 10 as well. After extensive feedback, testing and a number of new Ext4 changes developed in coordination with the lead Red Hat filesystem developer, Eric Sandeen (who hacks on Ext4 as well as XFS), Ext4 is now enabled as the default filesystem in Fedora 11.

Does GRUB support Ext4?

There is a patch in Red Hat Bugzilla to handle this. It has not been merged however for this release cycle due to lack of time for comprehensive testing and prioritization of other more important fixes. Since Fedora uses LVM by default and GRUB doesn't support it either, in practise, you will still need to continue using Ext3 filesystem for /boot partition. It is the recommended setup and that's how Anaconda sets it up by default. Anaconda will not let you format a /boot partition with Ext4 because of this.

I heard about data loss issues with delayed allocation and patches for that. Does Fedora include them?

Yes. They are headed upstream for 2.6.30 kernel and Fedora has backported those fixes to the 2.6.29 kernel in Fedora 11. They have been in the Rawhide development branch for the last few months now.

References:

http://lwn.net/Articles/326708/

http://lwn.net/Articles/322823/

How do I migrate from Ext3 to Ext4?

You can use the Anaconda boot option, "ext4migrate" to automatically migrate from Ext3 to the new Ext4 filesytem when upgrading from a previous release. If you do not use this option, Ext3 will be retained as your default filesystem

Does Fedora still offer the choice of Ext3?

Yes. The regular DVD install or network boot images still lets you choose either Ext4, Ext3 or XFS. The Live CD/DVD images however use Ext4 (except for /boot formatted as Ext3) and essentially transfer the entire image into the hard disk as part of the installation process and does not offer the ability to format the hard disk with any other filesystem. If you must choose a different filesystem, the regular DVD install or network boot images are the recommended alternatives.

What about backward/forward compatibility?

The on-disk format (extends) for Ext4 is new and incompatible with the Ext3 on-disk format. Ext4 provides a number of performance and scalability improvements that were not possible with the older format used by Ext3. You can however use the Ext4 option to mount any Ext3 partition. Any new files will use the new format by default. If you want to get the other improvements from Ext4 but without changing the disk format, you can use the no extends mount option. You can also mount with the no journal option to be compatible with Ext2. Refer to the man page for more details.

If you want to immediately take advantage of all the improvements, you must backup your data and reformat your disk with the new filesystem.

Has Fedora changed the Ext3 default mount options ?

No. In 2.6.30 kernel, the default option has been switched from ordered to writeback. Since Fedora includes the 2.6.29 kernel, there is no change in the default option. There are no plans to change it in the future either.

Reference:

http://lwn.net/Articles/328363/