Ext4 defrag file

It swaps the extents atomically if the inode has not changed between the start of the data copy and the completion of it. Hence you can't defragment active files. This was considered a fundamental blocker for ext4 even though most active files never need defragmentation think shared libraries. Hence the ext4 patchset implements data movement inside ext4 itself and so the kernel defrag code is much, much more complex than the XFS swap extents ioctl. Userspace complexity is about the same, but different APIs were required for ext4 to do it's "a bit at a time" algorithm If you know that you are overwriting the entire stripe there is no need to read any old data first, just calculate the new parity and do the write.

LILO does not support ext4, if I'm not mistaken. Not even GRUB supports it in stable releasses. User: Password:. Online defragmentation for ext4. By Jonathan Corbet February 4, Btrfs is currently getting around this by dropping bmap support, so swapfiles on btrfs won't work at all.

A real long term solution is required ;. Posted Feb 5, UTC Thu by brouhaha subscriber, [ Link ] While it's obviously possible, I've never had any serious problem with the swap file getting extremely fragmented.

It would be fine by me if online defragmentation of the swap file wasn't allowed. Instead of building complicated mechanisms for file systems to support that, and requiring file systems to use it, a relatively simple piece of kernel code could check whether the file in question was an active swap file, and deny the request from user space.

Posted Feb 5, UTC Thu by amarjan guest, [ Link ] Indeed, and there's a utility for Windows called pagedefrag that will defragment the windows pagefile and other critical system files early on boot, before the system is up and using them. Has anybody mentioned why online defragmentation ext4 can't just use the same interfaces? Online defragmentation for ext4 Posted Feb 5, UTC Thu by mp subscriber, [ Link ] If I understand correctly, there is also one other reason, not mentioned clearly in the article, why the defragmentation daemon needs some support from the kernel space.

The "put the newly defragmented version in the place of the old one" part must be done by updating the inode of the original or you would end up having to hunt and update all the directory entries pointing to the file.

You might also wonder whether ext4 is still in active development at all, given the flurries of news coverage of alternate filesystems such as btrfs, xfs, and zfs. Our latest Linux articles We can't cover everything about filesystems in a single article, but we'll try to bring you up to speed on the history of Linux's default filesystem, where it stands, and what to look forward to.

I drew heavily on Wikipedia's various ext filesystem articles, kernel. Andrew Tannenbaum developed it for teaching purposes and released its source code in print form!

Still, this was incredibly inexpensive for the time, and MINIX adoption took off rapidly, soon exceeding Tannenbaum's original intent of using it simply to teach the coding of operating systems. But wait, this is a filesystem article, right?

In , the typical hard drive was already MB in size. Linux clearly needed a better filesystem! First implemented in —only a year after the initial announcement of Linux itself! But ext didn't have a long reign, largely due to its primitive timestamping only one timestamp per file, rather than the three separate stamps for inode creation, file access, and file modification we're familiar with today.

A mere year later, ext2 ate its lunch. While ext still had its roots in "toy" operating systems, ext2 was designed from the start as a commercial-grade filesystem, along the same principles as BSD's Berkeley Fast File System. Ext2 offered maximum filesizes in the gigabytes and filesystem sizes in the terabytes, placing it firmly in the big leagues for the s. There were still problems to solve, though: ext2 filesystems, like most filesystems of the s, were prone to catastrophic corruption if the system crashed or lost power while data was being written to disk.

They also suffered from significant performance losses due to fragmentation the storage of a single file in multiple places, physically scattered around a rotating disk as time went on. Despite these problems, ext2 is still used in some isolated cases today—most commonly, as a format for portable USB thumb drives.

In , six years after ext2's adoption, Stephen Tweedie announced he was working on significantly improving it. This became ext3, which was adopted into mainline Linux with kernel version 2. If you lose power while writing data to the filesystem, it can be left in what's called an inconsistent state—one in which things have been left half-done and half-undone. This can result in loss or corruption of vast swaths of files unrelated to the one being saved or even unmountability of the entire filesystem.

Ext3, and other filesystems of the late s, such as Microsoft's NTFS, uses journaling to solve this problem.

The journal is a special allocation on disk where writes are stored in transactions; if the transaction finishes writing to disk, its data in the journal is committed to the filesystem itself.

If the system crashes before that operation is committed, the newly rebooted system recognizes it as an incomplete transaction and rolls it back as though it had never taken place. This means that the file being worked on may still be lost, but the filesystem itself remains consistent, and all other data is safe. Three levels of journaling are available in the Linux kernel implementation of ext3: journal , ordered , and writeback. Like ext2 before it, ext3 uses bit internal addressing. This means that with a blocksize of 4K, the largest filesize it can handle is 2 TiB in a maximum filesystem size of 16 TiB.

Theodore Ts'o who by then was ext3's principal developer announced ext4 in , and it was added to mainline Linux two years later, in kernel version 2. Ts'o describes ext4 as a stopgap technology which significantly extends ext3 but is still reliant on old technology. He expects it to be supplanted eventually by a true next-generation filesystem.

Ext4 is functionally very similar to ext3, but brings large filesystem support, improved resistance to fragmentation, higher performance, and improved timestamps. Ext4 was specifically designed to be as backward-compatible as possible with ext3. This not only allows ext3 filesystems to be upgraded in place to ext4; it also permits the ext4 driver to automatically mount ext3 filesystems in ext3 mode, making it unnecessary to maintain the two codebases separately. Ext3 filesystems used bit addressing, limiting them to 2 TiB files and 16 TiB filesystems assuming a 4 KiB blocksize; some ext3 filesystems use smaller blocksizes and are thus limited even further.

Ext4 uses bit internal addressing, making it theoretically possible to allocate files up to 16 TiB on filesystems up to 1,, TiB 1 EiB. Ext4 introduces a lot of improvements in the ways storage blocks are allocated before writing them to disk, which can significantly increase both read and write performance. An extent is a range of contiguous physical blocks up to MiB, assuming a 4 KiB block size that can be reserved and addressed at once.

Utilizing extents decreases the number of inodes required by a given file and significantly decreases fragmentation and increases performance when writing large files. Ext3 called its block allocator once for each new block allocated.

This could easily result in heavy fragmentation when multiple writers are open concurrently. However, ext4 uses delayed allocation, which allows it to coalesce writes and make better decisions about how to allocate blocks for the writes it has not yet committed. When pre-allocating disk space for a file, most file systems must write zeroes to the blocks for that file on creation. Ext4 allows the use of fallocate instead, which guarantees the availability of the space and attempts to find contiguous space for it without first needing to write to it.

This significantly increases performance in both writes and future reads of the written data for streaming and database applications. This is a chewy—and contentious—feature. Delayed allocation allows ext4 to wait to allocate the actual blocks it will write data to until it's ready to commit that data to disk. By contrast, ext3 would allocate blocks immediately, even while the data was still flowing into a write cache. Delaying allocation of blocks as data accumulates in cache allows the filesystem to make saner choices about how to allocate those blocks, reducing fragmentation write and, later, read and increasing performance significantly.

Introduction Ext4 is the evolution of the most used Linux filesystem, Ext3. In many ways, Ext4 is a deeper improvement over Ext3 than Ext3 was over Ext2. Ext3 was mostly about adding journaling to Ext2, but Ext4 modifies important data structures of the filesystem such as the ones destined to store the file data.

The result is a filesystem with an improved design, better performance, reliability and features. EXT4 features 2. Compatibility Any existing Ext3 filesystem can be migrated to Ext4 with an easy procedure which consists in running a couple of commands in read-only mode described in the next section.

If you need the advantages of Ext4 on a production system, you can upgrade the filesystem. The procedure is safe and doesn't risk your data obviously, backup of critical data is recommended, even if you aren't updating your filesystem :. This means, of course, that once you convert your filesystem to Ext4 you won't be able to go back to Ext3 again although there's a possibility, described in the next section, of mounting an Ext3 filesystem with Ext4 without using the new disk format and you'll be able to mount it with Ext3 again, but you lose many of the advantages of Ext4.

Ext4 adds bit block addressing, so it will have 1 EB of maximum filesystem size and 16 TB of maximum file size. Why bit and not bit? There are some limitations that would need to be fixed before making Ext4 fully bit capable, which have not been addressed in Ext4.

The Ext4 data structures have been designed keeping this in mind, so a future update to Ext4 will implement full bit support at some point. Note: The code to create filesystems bigger than 16 TB is -at the time of writing this article- not in any stable release of e2fsprogs. It will be in future releases.

Sub directory scalability Right now the maximum possible number of sub directories contained in a single directory in Ext3 is Ext4 breaks that limit and allows an unlimited number of sub directories. Extents The traditionally Unix-derived filesystems like Ext3 use an indirect block mapping scheme to keep track of each block used for the blocks corresponding to the data of a file.

Modern filesystems use a different approach called "extents". An extent is basically a bunch of contiguous physical blocks. It basically says "The data is in the next n blocks". For example, a MB file can be allocated into a single extent of that size, instead of needing to create the indirect mapping for blocks 4 KB per block.

Huge files are split in several extents. Extents improve the performance and also help to reduce the fragmentation, since an extent encourages continuous layouts on the disk. AFAIK it is now under review. Is it possible for me to e. I have many relevant reasons to believe the fragmentation on these machines is slowing down the read speed, example:.

As you can see, the defragmentation was not even able to put the 31 blocks file into 1 piece. Of course you might argue it is a movie file, so it does not matter. True, but only in this case. As for free space defragmentation and relevant file defragmentation, the patches were never completed; the last mention on the relevant mailing list dates back to :.

The e4defrag is in e2fsprogs , and the code is still getting maintained and improved. He recently also sent a code refactor of the kernel code which significantly improved it and shrank the size of ext4 by lines of code.

That being said, there hasn't been any real feature development for e4defrag in quite some time. There has been some discussion about what the kernel APIs might be to support this feature, but there has never been a finalized API proposal, let alone an implementation. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

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