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ext4 (the fourth extended filesystem) is a Linux filesystem format that organizes storage into blocks and block groups. Its journal helps recover committed filesystem metadata transactions after a crash, but the default mode does not journal all file contents. That distinction matters: ext4 journaling can help keep filesystem structure consistent, but it is not a backup and does not guarantee that recently written file data survives a crash.
How ext4 organizes storage
Ext4 divides a filesystem into blocks, then groups those blocks into block groups. Its allocator tries to place a file’s blocks near one another, often within the same group, to reduce fragmentation-related performance problems. The Linux kernel’s ext4 overview describes 4 KiB as a typical block size; at that size, a block group contains 32,768 blocks, or 128 MiB. Linux kernel documentation: High Level Design
Those layout figures describe the documented format, not a promise about every installation. Actual configuration and supported limits depend on the filesystem format, kernel, utilities, and device.
What ext4 journaling does—and does not—protect
Ext4’s journal records committed transactions. Following a crash, recovery can replay committed transactions so a metadata change is not left only partly applied. The journal therefore helps protect filesystem metadata consistency; it does not make every file’s contents crash-proof.
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The Linux kernel documentation puts the default behavior plainly: “For performance reasons, ext4 by default only writes filesystem metadata through the journal.” In default data=ordered mode, related data blocks are written before their metadata is committed, but file data itself is not fully journaled. A crash can still leave data in files written shortly beforehand incomplete or inconsistent. Linux kernel documentation: Journal (jbd2)
Journaling is not a substitute for backups. It is intended to support filesystem recovery after a crash, not to restore files deleted by a user, recover from device failure, or guarantee that all recent writes are intact.
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Ext4 data modes compared
| Mount mode | What is journaled | Crash and performance trade-offs in kernel documentation |
|---|---|---|
data=writeback |
Metadata; file data is not journaled. | The guide says this typically offers ext4’s best performance. After a crash, recovery can leave incorrect data in files written shortly before the crash. |
data=ordered (default) |
Metadata; related data blocks are written before the metadata. | Generally slower than writeback and significantly faster than journal mode. File data is not fully journaled. |
data=journal |
Both file data and metadata are written through the journal. | Generally the slowest mode. The guide says it disables delayed allocation and O_DIRECT support. |
These are trade-offs described by the Linux kernel’s ext4 guide, not workload-specific benchmark results. The guide does not establish that ext4 as a whole is faster or slower than another filesystem for a particular workload. Linux kernel documentation: ext4 General Information
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How large can an ext4 filesystem or file be?
There is no single maximum that applies to every ext4 setup. The kernel’s documented limits vary by block size, 32-bit versus 64-bit support, and whether a file uses extents or block maps. For 4 KiB blocks, the documented filesystem limit is 16 TiB in the 32-bit case and 64 ZiB in the 64-bit case; the extent-based file limit is 16 TiB in both tables. These are format limits, not guarantees that a particular kernel, toolset, or device can create or use a filesystem at that size. Linux kernel documentation: Blocks
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The on-disk-format documentation labels itself a work in progress and says its definitions reflect a Linux 4.18 and e2fsprogs 1.44 baseline. That scope is a stated baseline for the document, not an indication of current release versions. Linux kernel documentation: About this Book
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What to remember when choosing or troubleshooting ext4
- Think of the journal as a way to recover filesystem metadata transactions, not as a complete record of all file contents.
- For an actual size or compatibility question, check the target system’s kernel and filesystem tools alongside the block size and format features; do not rely on a headline maximum alone.
- Mount-option behavior can be version-sensitive. For example, the kernel guide says the
discard/nodiscardoption controls whether freed blocks issue discard commands to the underlying device and that discard is off by default on the documented page. Check documentation for the kernel you use before changing this setting. Linux kernel documentation: ext4 General Information
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