Migrating to ext4

Ext4 is the latest in a long line of Linux® file systems, and it's likely to be as important and popular as its predecessors. As a Linux system administrator, you should be aware of the advantages, disadvantages, and basic steps for migrating to ext4. This article explains when to adopt ext4, how to adapt traditional file system maintenance tool usage to ext4, and how to get the most out of the file system.

Current and upcoming features of ext4 that provide advantages over ext3

Feature	Advantage
Larger file systems	Ext3 tops out at 32 tebibyte (TiB) file systems and 2 TiB files, but practical limits may be lower than this depending on your architecture and system settings—perhaps as low as 2 TiB file systems and 16 gibibyte (GiB) files. Ext4, by contrast, permits file systems of up to 1024 pebibyte (PiB), or 1 exbibyte (EiB), and files of up to 16 TiB. This may not be important (yet!) for the average desktop computer or server, but it is important to users with large disk arrays.
Extents	An extent is a way to improve the efficiency of on-disk file descriptors, reducing deletion times for large files, among other things.
Persistent preallocation	If an application needs to allocate disk space before actually using it, most file systems do so by writing 0s to the not-yet-used disk space. Ext4 permits preallocation without doing this, which can improve the performance of some database and multimedia tools.
Delayed allocation	Ext4 can delay allocating disk space until the last moment, which can improve performance.
More subdirectories	If you've ever felt constrained by the fact that a directory can only hold 32,000 subdirectories in ext3, you'll be relieved to know that this limit has been eliminated in ext4.
Journal checksums	Ext4 adds a checksum to the journal data, which improves reliability and performance.
Online defragmentation	Although ext3 isn't prone to excessive fragmentation, files stored on it are likely to become at least a little fragmented. Ext4 adds support for online defragmentation, which should improve overall performance.
Undelete	Although it hasn't been implemented yet, ext4 may support undelete, which, of course, is handy whenever somebody accidentally deletes a file.
Faster file system checks	Ext4 adds data structures that permit fsck to skip unused parts of the disk in its checks, thus speeding up file system checks.
Nanosecond timestamps	Most file systems, including ext3, include timestamp data that is accurate to a second. Ext4 extends the accuracy of this data to a nanosecond. Some sources also indicate that the ext4 timestamps support dates through April 25, 2514, versus January 18, 2038, for ext3.

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