Ext4 (Italiano)

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Notes: please use the first argument of the template to provide more detailed indications. (Discuss in Talk:Ext4 (Italiano)#)

Ext4 è l'evoluzione del filesystem Linux più utilizzato, ext3. Per molti versi, ext4 è un miglioramento più consistente di Ext3 rispetto quello che è stato ext3 su ext2. Ext3 implementava la nuova funzionalità del journaling a Ext2, ma Ext4 modifica importanti strutture dati nel filesystem, come quelli destinati a memorizzare i file dei dati. Il risultato è un filesystem con un design migliorato, migliori prestazioni, affidabilità e funzionalità.

Fonte: Ext4 - Linux Kernel Newbies

Creating ext4 Partitions From Scratch

  1. Upgrade your system: Template:Codeline
  2. Format the partition: Template:Codeline (replace Template:Codeline with the device to format (e.g. Template:Codeline))
  3. Mount the partition
  4. Add an entry to Template:Filename, using the filesystem 'type' ext4
Tip: See the mkfs.ext4 man page for more options; edit Template:Filename to view/configure default options.

Migrating From ext3 to ext4

There are two ways of migrating partitions from ext3 to ext4:

  • mounting ext3 partitions as ext4 without converting (compatibility)
  • converting ext3 partitions to ext4 (performance)

These two approaches are described below.

Mounting ext3 Partitions as ext4 Without Converting

Rationale

A compromise between fully converting to ext4 and simply remaining with ext3 is to mount existing ext3 partitions as ext4.

Pros:

  • Compatibility (the filesystem can continue to be mounted as ext3) – This allows users to still read the filesystem from other distributions/operating systems without ext4 support (e.g. Windows with ext3 drivers)
  • Improved performance (though not as much as a fully-converted ext4 partition) – See Ext4 - Linux Kernel Newbies for details

Cons:

  • Fewer features of ext4 are used (only those that do not change the disk format such as multiblock allocation and delayed allocation)
Note: Except for the relative novelty of ext4 (which can be seen as a risk), there is no major drawback to this technique.

Procedure

  1. Edit Template:Filename and change the 'type' from ext3 to ext4 for any partitions you would like to mount as ext4.
  2. Re-mount the affected partitions.
  3. Done.

Converting ext3 Partitions to ext4

Rationale

To experience the benefits of ext4, an irreversible conversion process must be completed.

Pros:

Cons:

  • Read-only access from Windows can be provided by Ext2Explore, but there is currently no driver for writing data.
  • Irreversible (ext4 partitions cannot be 'downgraded' to ext3)

Prerequisites

The following software is required on the Arch Linux system:

If converting one's /boot partition to ext4:

Note: The ext4 patch is included by default with Arch's GRUB package (at the time of writing, but this will likely not change). Otherwise, GRUB2 is required for booting from an ext4 partition.
Warning: Booting from an ext4 partition is not 'officially' supported by GRUB, and GRUB2 is still under development. While GRUB does currently work, the 'safe' option is to boot from an ext2 or ext3 /boot partition. CONSIDER YOURSELF WARNED!

If converting one's root (/) partition to ext4:

If converting one's root (/) partition to ext4, the following software is also needed on a bootable CD/USB drive:

Note: Using the latest Arch Linux release (2009.02) is recommended. Older Arch Linux images (<= 2008.06) ship with an older version of Template:Codeline, but it is a simple matter to Template:Codeline from the live environment after setting up networking. Alternatively, SystemRescueCd >= 1.1.4 contains an appropriate version, and is in itself a handy CD to have.

Procedure

These instructions were adapted from http://ext4.wiki.kernel.org/index.php/Ext4_Howto and http://bbs.archlinux.org/viewtopic.php?id=61602. They have been tested and confirmed by this author as of January 16, 2009.

  • UPGRADE! Perform a sysupgrade to ensure all required packages are up-to-date: Template:Codeline
  • BACK-UP! Back-up all data on any ext3 partitions that are to be converted to ext4. Although ext4 is considered 'stable' for general use, it is still a relatively young and untested file system. Furthermore, this conversion process was only tested on a relatively simple setup; it is impossible to test each of the many possible configurations the user may be running.
  • Edit Template:Filename and change the 'type' from ext3 to ext4 for any partitions that are to be converted to ext4.
Warning: ext4 is backwards-compatible with ext3 until extents and other new fancy options are enabled. If the user has a partition that is shared with another OS that cannot yet read ext4 partitions, it is possible to mount said partition as ext4 in Arch and still be able to use it as ext3 elsewhere at this point... Not so after the next step! Note, however, that there are fewer benefits to using ext4 if the partition is not fully converted.
  • The conversion process with Template:Codeline must be done when the drive is not mounted. If converting one's root (/) partition, the simplest way to achieve this is to boot from some other live medium, as described in the 'Prerequisites' section above.
Note: The user MUST fsck the filesystem, or it will be unreadable! This fsck run is needed to return the filesystem to a consistent state. It WILL find checksum errors in the group descriptors -- this is expected. The '-f' parameter asks fsck to force checking even if the file system seems clean. The '-p' parameter asks fsck to 'automatically repair' (otherwise, the user will be asked for input for each error). You may need to run fsck -f rather than fsck -fp.
  • Reboot Arch Linux!
Warning: If the user converted their root (/) partition, a kernel panic may be encountered when attempting to boot. If this happens, simply reboot using the 'fallback' initial ramdisk and re-create the 'default' initial ramdisk: Template:Codeline

Migrating files to extents

Even though the filesystem is now converted to ext4, all files that have been written before the conversion do not yet take advantage of the new extents of ext4, which will improve large file performance and reduce fragmentation and filesystem check time. In order to fully take advantage of ext4, all files would have to be rewritten on disk. A utility called e4defrag is being developed and will take care of this task ; however, it is not yet ready for production.

Fortunately, it is possible to use the chattr program, which will cause the kernel to rewrite the file using extents. It is possible to run this command on all files and directories of one partition (e.g. if /home is on a dedicated partition):

find /home -xdev -type f -print0 | xargs -0 chattr +e
find /home -xdev -type d -print0 | xargs -0 chattr +e

It is recommended to test this command on a small number of files first, and check if everything is going all right. It may also be useful to check the filesystem after conversion.

Using the lsattr command, it is possible to check that files are now using extents. The letter 'e' should appear in the attribute list of the listed files.

Troubleshooting

Kernel Panic

One problem this author encountered was a kernel panic after converting the root (/) partition to ext4. This is because the initial ramdisk was detecting the partition as 'ext4dev', rather than 'ext4'. It was a simple matter to boot with the 'fallback' initial ramdisk and re-create the 'default' initial ramdisk :

  • Remount the root partition in read-write mode; assuming 'XXX' is your root partition :
# mount /dev/XXX / -o remount,rw
  • Manually mount the boot partition on /boot if it is on a separate partition.
  • Re-create the ramdisk :
# mkinitcpio -p kernel26

During the creation process, Template:Codeline correctly detected and included ext4 modules in the initial ramdisk.

GRUB Error 13

After a recent kernel update, this author encountered a GRUB error while attempting to boot from an ext4 /boot partition:

Error 13: Invalid or unsupported executable format

The solution is to boot from the live medium and chroot into the Arch Linux installation:

# mkdir /mnt/arch
# mount -t ext4 /dev/sda1 /mnt/arch
# mount -t proc proc /mnt/arch/proc
# mount -t sysfs sys /mnt/arch/sys
# mount -o bind /dev /mnt/arch/dev
# chroot /mnt/arch /bin/bash

If /boot is on a separate partition, this partition must also be mounted:

# mount -t ext4 /dev/sda2 /boot

Then, the following command should resolve the issue. (Does anyone know why?):

# grub-install --recheck /dev/sda

Data Corruption

Some early adopters of ext4 encountered data corruption after a hard reboot. Please read Ext4 data loss; explanations and workarounds for more information.

Since kernel 2.6.30, ext4 is considered "safe(er)." Several patches improved the robustness of ext4 - albeit at a slight performance cost. A new mount option (Template:Codeline) can be used to disable this behavior. For more information, please read Linux 2 6 30 - Filesystems performance improvements.

For kernel versions earlier than 2.6.30, consider adding Template:Codeline to the Template:Codeline line in GRUB's Template:Filename as a preventative measure.

Improving performance

Since kernel 2.6.30, ext4 performance has decreased due to changes that serve to improve data integrity.[1] Users can improve performance with the Template:Codeline option when mounting the disk, but this can be dangerous and may result in data loss or corruption after power failures. To turn barriers off, add the option Template:Codeline to the desired filesystem in Template:Filename. For example:

# /dev/sda5    /    ext4    noatime,barrier=0    0    1