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 Forum index » Advanced Topics » Cutting edge
ext4 File System?
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Joined: 10 Mar 2007
Posts: 208

PostPosted: Fri 29 Feb 2008, 09:46    Post_subject:  ext4 File System?  

Anyone thinking of implementing ext4 in Puppy? Some notes on ext4 follow:

From Wikipedia:

Large filesystem

The ext4 filesystem can support volumes with sizes up to 1 exbibyte (1024 pebibytes).

[edit] Extents

Extents are introduced to map a range of contiguous physical blocks into a single descriptor. A single extent can map up to 128MiB of contiguous space with a 4KiB block size.[6]

[edit] Backward compatibility

The ext4 filesystem is backward compatible with ext3, making it possible to mount an ext3 filesystem as ext4 (using the “ext4dev” filesystem type).

[edit] Forward compatibility

The ext4 filesystem is forward compatible with ext3, that is, it can be mounted as an ext3 partition (using “ext3” as the filesystem type when mounting). However, if the ext4 partition uses extents (one of the major new features of ext4), forward compatibility and therefore the ability to mount the filesystem as ext3 is lost. Extents were enabled by default in the 2.6.23 kernel. Previously, the “extents” option was explicitly required (e.g. mount /dev/sda1 /mnt/point -t ext4dev -o extents).

[edit] Persistent pre-allocation

The ext4 filesystem allows for pre-allocation of on disk space for a file. The current methodology for this on most file systems is to write the file full of 0's to reserve the space when the file is created (although XFS has an ioctl to allow for true pre-allocation as well). This method would no longer be required for ext4. The space allocated for files such as these would be guaranteed and would likely be contiguous. This has applications for media streaming and databases.

[edit] Delayed allocation

Main article: Allocate-on-flush

Ext4 uses a "delayed allocation" feature to delay block allocation as long as possible. During this delay pending writes will only change the free space counter. This improves performance and reduces fragmentation.

[edit] Break 32000 subdirectory limit

In ext3 the number of subdirectories that a directory could contain was limited to 32000. This limit is lifted in ext4. To allow for continued performance given the possibility of much larger directories, Htree indexes (a specialized version of a Btree) is turned on by default in ext4. This feature is implemented in 2.6.23. Htree is also available in ext3 with dir_index enabled.

[edit] Journal checksumming

Ext4 uses checksums in the journal to improve reliability, since the journal is one of the most used parts of the disk. This feature has a side benefit; it can safely avoid a phase of the journaling process, improving performance slightly.

[edit] Online defragmentation

Ext4 will also have an online defragmenter. Even with the various techniques used to avoid it, a long lived file system does tend to become fragmented over time. Ext4 has a tool which can defragment individual files or entire file systems.

[edit] Faster file system checking

In ext4, unallocated block groups and sections of the inode table are marked as such. This enables e2fsck to skip them entirely on a check and greatly reduce the time it takes to check a file system of the size ext4 is built to support. This feature is Implemented in 2.6.24.

[edit] Nanosecond timestamps

As computers become faster in general and specifically Linux becomes used more for mission critical applications, the granularity of second-based timestamps becomes insufficient. To solve this, ext4 will have timestamps measured in nanoseconds. This feature is currently implemented in 2.6.23.

[edit] References
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Joined: 26 May 2006
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Location: Southampton, UK

PostPosted: Fri 29 Feb 2008, 13:46    Post_subject: Re: ext4 File System?  

friedsonjm wrote:
The ext4 filesystem can support volumes with sizes up to 1 exbibyte (1024 pebibytes).
Oh good, I've got a couple of those lying around, having to split them up into 32 terrabyte partitions was really starting to irritate me Wink
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Joined: 04 May 2005
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PostPosted: Sat 01 Mar 2008, 00:58    Post_subject:  

Great, another new word to learn. I'm running out of memory. Laughing

Rather than use names I've never seen before, and mostly are beloved by salesmen because they are impressive, if inaccurate and misleading, why not just use the power of 2? An exbibyte is equal to 2 raised to the (what) power? Confused
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