Talk:Extent (file systems)

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On the "Comparison of File Systems" page, Apple's HFS+ filesystem is listed as supporting Extents. However it's not listed on this page -- perhaps it should be? I'm not familar enough with the system to know whether it's true or not, so I won't add it myself, but maybe somebody else can confirm or deny and rectify the discrepency. It seems that one page or the other needs to be changed. -- Kadin2048 15:38, 6 January 2006 (UTC)

How is allocating an extent to a file different to pre-allocating a contiguous series of blocks, if the file-system supports packing more than one file into an extent... and how does extent allocation differ from just using a larger block size? Martin Rudat(T|@|C) 08:56, 18 February 2006 (UTC)

At least as I see it, a file system that "supports extents" has "file maps" (the "file map" being the per-file data structure used to map from offsets in the file to blocks on disk) that can contain entries that can map a variable-length range of file offsets to a contiguous sequence of disk blocks, while a file system that doesn't "support extents" has file maps that have entries that map a fixed-size range of offsets to a fixed-size contiguous sequence of disk blocks.

With that model, a larger block size for a file system that doesn't "support extents" just increases the size of the fixed-size ranges of offsets; a file system that "supports extents" can allocate extents with different sizes to the same file. You don't need extents to support pre-allocating a contiguous series of blocks, just to allow a single file map entry to point to the entire contiguous series.

Are there any file systems that support packing more than one file into an extent? That'd probably be useful only for files (or file tails) smaller than the minimum size of an extent; otherwise, you might as well split the extent into multiple smaller extents and assign each of those smaller extents in its entirety to the file whose data it contains. Guy Harris 23:33, 28 October 2006 (UTC)

In a FS without extents, if you allocate x contiguous blocks for a file and the file needs to grow beyond those x blocks, there may be the case where you simply can't let it grow, or it would overwrite other used blocks.

By using extents, you can allocate another chunk of contiguous space, located somewhere else, and thus solve this problem.

This concept can be found in Silberschatz's "Operating System Concepts", and i think it's not explaind clearly (or at all) in the article. Should I edit it? --Asymmetric (talk) 14:15, 13 April 2008 (UTC)

Contents 1 extent 2 HPFS 3 VTOCs on MVS, z/OS, etc 4 NTFS

[edit] extent

Shouldn't the definition of "extent" include "the entire scope of the boundaries (of something)"? Shortylumber 17:28, 5 September 2006 (UTC)shortylumber

[edit] HPFS

HPFS can preallocate space before a write. Is that the same?

[edit] VTOCs on MVS, z/OS, etc

The Volume Table of Contents (VTOC) on an MVS volume, describes space assigned to data sets (files, in more common terminology), free space, and even its own size in terms of extents.

Rlhamil 16:27, 26 March 2007 (UTC)

Ext4 is missing here! —Preceding unsigned comment added by 80.223.145.74 (talk) 15:25, 17 December 2007 (UTC)

[edit] NTFS

Is it really valid to list NTFS here? The described method requires the developer to perform file writes in a specific manner to make this happen. In my opinion this doesn't make NTFS qualify as an extent based file system. I believe that in order for a FS to qualify it must perform this task automatically; if we're going to allow this as part of the definition then nearly any file system should be able to make it to this list. For example, one could make nearly any file system pre-allocate space by using comparable calls. It should also be noted that the specified Win32 API calls do NOT guarantee that this space be contiguous! The system will try to make the space contiguous but it might not be possible, so I really believe that by these standards literally any file system could be put on this list, and therefore NTFS should be removed. —Preceding unsigned comment added by 24.222.116.189 (talk) 11:51, 2 January 2008 (UTC)

I just wanted to add that it looks like NTFS was removed from the list (probably rightly so), but the file system matrix on the following wiki page still shows the contrary: http://en.wikipedia.org/wiki/Comparison_of_file_systems --May 8, 2008 —Preceding unsigned comment added by 99.201.204.33 (talk) 18:00, 8 May 2008 (UTC)