1. 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 .

2. EXT4 features

2.1. 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). This means that you can improve the performance, storage limits and features of your current filesystems without reformatting and/or reinstalling your OS and software environment. If your 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 :). Ext4 will use the new data structures only on new data, the old structures will remain untouched and it will be possible to read/modify them when needed. This means, that, 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 a 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)

2.2. Bigger filesystem/file sizes

Currently, Ext3 support 16 TB of maximum filesystem size, and 2 TB of maximum file size. Ext4 adds 48-bit block addressing, so it will have 1 EB of maximum filesystem size and 16 TB of maximum file size. 1 EB = 1,048,576 TB (1 EB = 1024 PB, 1 PB = 1024 TB, 1 TB = 1024 GB). Why 48-bit and not 64-bit? There're some limitations that would need to be fixed before making Ext4 fully 64-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 64-bit support at some point. 1 EB will be enought (really :) until that happens.

2.3. Subdirectory scalability

Right now the maximum possible number of subdirectories contained in a single directory in Ext3 is 32.000. Ext4 breaks that limit and allows a unlimited number of subdirectories.

2.4. Extents

The traditionally Unix-derived filesystems like Ext3 use a indirect block mapping scheme to keep track of each block used for the blocks corresponding to the data of a file. This is inefficient for large files, specially on large file delete and truncate operations, because the mapping keeps a entry for every single block, and big files have many blocks -> huge mappings, slow to handle. Modern filesystems use a different approach called "extents". A extent is basically a bunch of contiguous physical blocks. It basically says "The data is in the next n blocks". For example, a 100 MB file can be allocated into a single extent of that size, instead of needing to created the indirect mapping for 25600 blocks (4 KB per block). Huge files are split in several extents. Extents improve the performance and also help to reduce the fragmentation, since a extent encourages continuous layouts on the disk.

2.5. Multiblock allocation

When Ext3 needs to write new data to the disk, there's a block allocator that decides which free blocks will be used to write the data. But the Ext3 block allocator only allocates one block (4KB) at a time. That means that if the system needs to write the 100 MB data mentioned in the previous point, it will need to call the block allocator 25600 times (and it was just 100 MB!). Not only this is inefficient, it doesn't allow the block allocator to optimize the allocation policy because it doesn't knows how many total data is being allocated, it only knows about a single block. Ext4 uses a "multiblock allocator" (mballoc) which allocates many blocks in a single call, instead of a single block per call, avoiding a lot of overhead. This improves the performance, and it's specially useful with delayed allocation and extents. This feature doesn't affect the disk format. Also, note that the Ext4 block/inode allocator has other improvements, described in detail [ in this paper].

2.6. Delayed allocation

[ Delayed allocation] is a performance feature (it doesn't change the disk format) found in a few modern filesystems such as XFS, ZFS, btrfs or Reiser 4, and it consists in delay the allocation of blocks as much as possible, contrary to what traditionally filesystems (such as Ext3, reiser3, etc) do: allocate the blocks as soon as possible. For example, if a process write()s, the filesystem code will allocate inmmediately the blocks where the data will be placed - even if the data is not being written right now to the disk and it's going to be kept in the cache for some time. This approach has disadvantages. For example when a process is writting continually to a file that grows, succesive write()s allocate blocks for the data, but they don't know if the file will keep growing. Delayed allocation, in the other hand, does not allocate the blocks inmmediately when the process write()s, rather, it delays the allocation of the blocks while the file is kept in cache, until it is really going written to the disk. This gives to the block allocator the oportunity to optimize the allocation in situations where the old system couldn't. Delayed allocation plays very nicely with the two previous features mentioned, extents and multiblock allocation, because in a many workloads when the file is written finally to the disk it will be allocated in extents whose block allocation is done with the mballoc allocator. The performance is much better, and the fragmentation is much improved in some workloads.

2.7. Fast fsck

Fsck is a very slow operation, specially the first step: checking all the inodes in the filesystem. In Ext4, at the end of each group's inode table it will be stored a list of unused inodes (with a checksum, for safety), so fsck will not check those inodes. The result is that total fsck time improves from 2 to 20 times, depending on the number of used inodes ( It must be noticed that it's fsck, and not Ext4, who will build the list of unused inodes. This means that you must run fsck to get the list of unused inodes built, and only the next fsck run will be faster (you need to pass a fsck in order to convert a Ext3 filesystem to Ext4 anyway). There's also a feature that takes part on this fsck speed up - "flexible block groups" - that also speeds up filesystem operations.

2.8. Journal checksumming

The journal is the most used part of the disk, making the blocks that form part of it more prone to hardware failure. And recovering from a corrupted journal can lead to massive corruption. Ext4 checksums the journal data to know if the journal blocks are failing or corrupted. But journal checksumming has a bonus: it allows to convert the two-phase commit system of Ext3's journalling to a single phase, speeding the filesystem operation up to 20% in some cases - so reliability and performance are improved at the same time.

2.9. Online defragmentation

(This feature is not available in 2.6.28, but will be probably available in the next release). While delayed allocation, extents and multiblock allocation help to reduce the fragmentation, with usage filesystems can still fragment. For example: You write three files in a directory and contigually on the disk. Some day you need to update the file of the middle, but the updated file has grown a bit, so there's no enought room for it. You have no option but fragment the excess of data to another place of the disk, which will cause a seek, or allocate the updated file contigually in another place, far from the other two files, resulting in seeks if an application needs to read all the files on a directory (say, a file manager doing thumbnails on a directory full of images). Besides, the filesystem can only care about certains types of fragmentation, it can't know, for example, that it must keep all the boot-related files contiguous, because it doesn't know which files are boot-related. To solve this issue, Ext4 will support online fragmentation, and there's a e4defrag tool which can defragment individual files or the whole filesystem.

Larger inodes, nanosecond timestamps, fast extended attributes, inodes reservation...

2.11. Persistent preallocation

This feature, available in Ext3 in the latest kernel versions, and emulated by glibc in the filesystems that don't support it, allows applications to preallocate disk space: Applications tell the filesystem to preallocate the space, and the filesystem preallocates the necessary blocks and data structures, but there's no data on it until the application really needs to write the data in the future. This is what P2P applications do in their own when they "preallocate" the neccesary space for a download that will last hours or days, but implemented much more efficiently by the filesystem and with a generic API. This have several uses: first, to avoid applications (like P2P apps) doing it themselves inneficiently by filling a file with zeros . Second, to improve fragmentation, since the blocks will be allocated at one time, as contiguously as possible. Third, to ensure that applications has always the space they know they will need, which is important for RT-ish applications, since without preallocation the filesystem could get full in the middle of an important operation. The feature is available via the libc posix_fallocate() interface.

2.12. Barriers on by default

This is an option that improves the integrity of the filesystem at the cost of some performance (you can disable it with "mount -o barrier=0", recommended trying it if you're benchmarking). From [ this LWN article]: "The filesystem code must, before writing the [journaling] commit record, be absolutely sure that all of the transaction's information has made it to the journal. Just doing the writes in the proper order is insufficient; contemporary drives maintain large internal caches and will reorder operations for better performance. So the filesystem must explicitly instruct the disk to get all of the journal data onto the media before writing the commit record; if the commit record gets written first, the journal may be corrupted. The kernel's block I/O subsystem makes this capability available through the use of barriers; in essence, a barrier forbids the writing of any blocks after the barrier until all blocks written before the barrier are committed to the media. By using barriers, filesystems can make sure that their on-disk structures remain consistent at all times."

3. How to use Ext4

This is the first stable version of Ext4, so even if the whole development and release of this filesystems has been slowed down and delayed a lot to guarantee the same level of stability that you'd expect from the current Ext3 implementation, the usual rules of any ".0" software apply.

One very important thing to keep in mind is that there is NOT Ext4 GRUB support. Well, that wasn't exactly true: There is grub support, but the grub versions used by your current distro don't support it. There's support in the GRUB2 development branch, but only from [ this commit] and ahead. There're available grub2 packages in Ubuntu and debian-derived distros as the grub-pc package. In the 0.9x branch, there's not official support, but there's a [ Google SoC] project that developed support for it, and Google finds [ patches]. So choose yourself. The next release of distros based in Linux 2.6.28 will probably have support in one way or another. The safe option is to keep your /boot directory in a partition formatted with Ext3. You also need an updated e2fsprogrs tool, of course...

Switching to Ext4 it's very easy. There're three different ways you can use to switch:

3.1. Creating a new Ext4 filesystem from the scratch

3.2. Migrate existing Ext3 filesystems to Ext4

You need to use the tune2fs and fsck tools in the filesystem, and that filesystem needs to be unmounted. Run:

After running this command you MUST run fsck. If you don't do it, Ext4 WILL NOT MOUNT your filesystem. This fsck run is needed to return the filesystem to a consistent state. It WILL tell you that it finds checksum errors in the group descriptors - it's expected, and it's exactly what it needs to be rebuilt to be able to mount it as Ext4, so don't get surprised by them. Since each time it finds one of those errors it asks you what to do, always say YES. If you don't want to be asked, add the "-p" parameter to the fsck command, it means "automatic repair":

There's another thing that must be mentioned. All your existing files will continue using the old indirect mapping to map all the blocks of data. The online defrag tool will be able to migrate each one of those files to a extent format (using a ioctl that tells the filesystem to rewrite the file with the extent format; you can use it safely while you're using the filesystem normally)

3.3. Mount an existing Ext3 filesystem with Ext4 without changing the format

You can mount an existing Ext3 filesystem with Ext4 but without using features that change the disk format. This means you will be able to mount your filesystem with Ext3 again. You can mount an existing Ext3 filesystem with "mount -t ext4 /dev/yourpartition /mnt". Doing this without having done the conversion process described in the previous point will force Ext4 to not use the features that change the disk format, such as extents, it will use only the features that don't change the file format, such as mballoc or delayed allocation. You'll be able to mount your filesystem as Ext3 again. But obviously you'll be losing the advanges of the Ext4 features that don't get used...

KernelNewbies: Ext4 (last edited 2008-12-22 22:51:01 by diegocalleja)