Talk:ext2

Strange Error Message

When I tried to access the ext2 article it returned the following error message: "Database error: A database query syntax error has occurred. This may indicate a bug in the software. The last attempted database query was:(SQL query hidden) from within function "LocalFile::upgradeRow". Database returned error "1223: Can't execute the query because you have a conflicting read lock (10.0.0.241)". Retrieved from "http://en.wikipedia.org/wiki/Ext2"

Anybody from Wikipedia know what this means? Public Menace (talk) 02:48, 5 July 2010 (UTC)

The site was down due to technical problems for a while. See [[WP:VPT|the Village pump (technical)}} page for an explanation.-gadfium 03:09, 5 July 2010 (UTC)

Maximum file size

The inode structure contains a i_blocks 32-bit value indicating the total number of 512-byte blocks reserved for the file content, regardless of the actual file size. The maximum size a file can thus attain is 2**32 * 512 bytes, or 2TB. —Preceding unsigned comment added by 199.212.150.16 (talk) 18:26, 19 May 2010 (UTC)

That's not quite right. i_blocks contain the number of (512-byte) blocks actually used by the file, but the file may be sparse and not use the full disk space its file size would imply. It is therefore quite possible to have very large file, even exceeding the size of the containing file system, so long as they have less than 2 TB of data blocks (and no more data blocks than the file system can contain). I seem to recall I created such files while testing filesystem software some years ago.-gadfium 21:07, 19 May 2010 (UTC)

An inode can also have blocks pre-allocated hence have more blocks assigned to it than currently used; the i_blocks always reflect the current number of blocks assigned to this inode; regarldess of whether these blocks are actually used or not for the file. I suspect that a standard fsck would complain if a file had a larger size than can fit in its i_blocks*512 without a compression extension configured. Ekscrypto (talk) 18:31, 25 May 2010 (UTC)

Volume size and kernel versions

I'm pretty sure the 2TiB limit only applies to 2.4 kernels and earlier, since that is the maximum block size (see, eg, [1]). I've edited to reflect this.

That's correct, actually this is documented in the Linux kernel sources under Documentation/Filesystems/Ext2.txt: There is a 2.4 kernel limit of 2048GB for a single block device, so no filesystem larger than that can be created at this time. There is also an upper limit on the block size imposed by the page size of the kernel, so 8kB blocks are only allowed on Alpha systems (and other architectures which support larger pages).

Ekscrypto (talk) 21:34, 3 February 2009 (UTC)

ctime

I changed the description of ctime to "change time of inode", and it was reverted on the basis of what ext2_fs.h says. I disagree with this, so I have put my change back again. — PhilHibbs | talk 12:16, 2 Mar 2005 (UTC)

actually ctime is "creation time" and mtime is "modified time". If "ctime" is "change time" then both mtime and ctime should by extrapolation be the same and they are not. Ekscrypto (talk) 21:47, 3 February 2009 (UTC)

Berkeley Fast File System background

The guys who wrote Unix System Administration Handbook page 151 mentions somewhere that Ext2 borrows its concepts from FFS. How true is this claim? After googling a bit, i came across a couple of links and it looks like they generalized it too far. See [2] specifically "ext2 file system is an intellectual descendant of the Berkeley Fast File System". I just mentioned it as it wouldn't hurt leveraging views from different sources.

(unsigned)

Ext2 is mostly a UFS filesystem with some VAX-era cruft stripped out.

UFS was designed for disks that actually used cylinder-head-sector addressing with a known interleave. You could set up UFS to be aware of this, avoiding some delays related to disk rotation. Such hacks are essentially impossible on modern hardware. When ext2 was being developed, UFS lost performance in a futile effort to optimize for something it could not know. Modern UFS doesn't try to play such games, but still retains the now-worthless on-disk data structures.

UFS traditionally used 8 K blocks with 1 K sub-blocks for file tails. More recently, the sizes are half of that. Ext2's normal block size range is 1 K, 2 K, or 4 K. Plans to implement fragments for Linux have been cancelled because the feature adds complexity and, unsurprisingly, fragmentation.

UFS has the 3 traditional UNIX file timestamps. Ext2 adds a fourth, the deletion time. This helps the filesystem checker to be more reliable. It costs an extra seek when a file is deleted.

Ext2 replicates the superblock in many places for extra reliability. During recovery from a disaster, the filesystem checker can locate a backup superblock if needed.

In the HURD variant, there is an author ID.

The file flags are a bit different.

Recent enhancements include ACLs, OS/2-style extended attributes (small named forks), security data tags, file type info in directory entries, growable filesystems, journalling (ext3), and hashed directories.

Non-standard kenhancements include compression, extent-based block allocation, phase-tree commits, and tree-structured tail packing.

The ext2 driver never seriously attempted to support sync operation. Despite this, recovery from crashes was generally superior to that of UFS because the ext2.fsck program was a truly excellent piece of software.

24.110.60.225 06:27, 1 January 2006 (UTC)

GNU & Kernel

Instead "used on the Linux operating system", how about "used in the Linux kernel". This is probably more accurate than "Linux operating system" or "GNU/Linux operating system". Jebba 06:51, 25 July 2005 (UTC) Also, this article is probably no longer a stub.

Design

The incomplete "Design" section was a bit of an eyesore. I'm moving its contents in here until I can find time for a full write-up. —Ghakko 22:35, 27 July 2005 (UTC)

Please note that with the introduction of sparse superblock in revision 1 of Ext2, not all block groups have a copy of the superblock and group descriptors. It should also be pointed throughout the design document the various changes between Revision 0 and Revision 1 of ext2 has it had impacts on inode size, extended attributes and many other features. Ekscrypto (talk) 21:50, 3 February 2009 (UTC)

Block group

Every ext2 filesystem is divided up into block groups, each of which has its own:

• Superblock
• Group descriptors
• Block bitmap
• Inode bitmap
• Inode table
• Data blocks

Block groups are similar to cylinder groups in Berkeley FFS, where they were introduced to retard file fragmentation.

Superblock

The superblock contains metadata describing the entire filesystem:

• Number of inodes
• Number of blocks
• Reserved block count
• First data block number
• Block size
• Fragment size
• Number of blocks per group
• Number of fragments per group
• Number of inodes per group
• Mount time
• Write time
• Mount count
• Maximal mount count
• Magic signature
• File system state: valid (unmounted cleanly) or error
• Error detection behavior
• Minor revision level
• Time the file system was last checked
• Maximum time between checks
• Default user & group ID for reserved blocks

Inodes

Inodes contain file metadata:

• File mode
• User ID (uid)
• Group ID (gid)
• File size, in bytes
• Last file access time (atime)
• Inode change time (ctime)
• Last file modification time (mtime)
• Deletion time (dtime)
• Link count
• Block count
• File type
• File attributes
• List of block addresses, each of which may to:
  • A block containing raw file data (direct address)
  • An indirect block containing direct addresses (indirect address)
  • A doubly-indirect block containing indirect addresses (doubly-indirect address)
  • A triply-indirect block containing doubly-indirect addresses (triply-indirect address)

Directories

Represented as a list of directory entries, each of which contains an inode number and a filename.

Files

Regular files

Represented as a list of blocks, pointers for which go into indirect nodes. Blocks are headerless and contain raw data.

Links

• Hard links
• Symbolic links

Special files

• Block devices
• Character devices
• FIFOs

Filename length

The article claims "maximum filename length of 255 characters" , while the article Comparison_of_file_systems claims "255 bytes".

xerces8 --86.61.44.206 21:22, 4 March 2006 (UTC)

A byte is the same size as a character so it is the same thing is it not? -Billy Leo

I think it just depends on what you define a character as. Strictly speaking, I'm pretty sure it's any byte except NULL. This set is not ASCII nor UTF-8 since 127 through 159 are not permitted in UTF-8 and ASCII only uses the first 7 bits. So I guess I don't know what character set your would define it as, which would lead me to say call it 255 bytes. Cburnett 03:20, 13 March 2006 (UTC)

Linux expects ISO-Latin-1 characters in filenames which uses 1 byte per character; but regardless of whether future support for full UTF-8, due the 8bit name_len record in the inode any filename is limited to 255 bytes. Ekscrypto (talk) 21:29, 3 February 2009 (UTC)

Article Title

To my knowledge, the official name for ext2 is the Second Extended Filesystem. Please correct me if i'm wrong, but shouldn't we change the article title to that? -- De Zeurkous (zeurkous@lichee.nichten.info), Mon Mar 20 17:03:48 UTC 2006

WP has a naming convention to use the common name over the "official" name. ext2 is definitely the most common. Cburnett 02:34, 21 March 2006 (UTC)

How It Works

I like the idea of going into more detail about what makes ext2 special (features, performance, whatever), but just adding a section header won't magically cause the rest of the internet to fill in the section. Crag 15:48, 21 November 2006 (UTC)

Proposal and argument in favour of removing redirection of debugfs to ext2fs

Hi,

I have removed the redirection of debugfs to this entry due to the differences between both of them. As I have written in the entry on debugfs and gathered from the references cited on the same page, debugfs is a kernel-space user-space interface and has no existence on the disk and hence should not be confused with ext2fs. Basically, it exists inside the RAM and is manipulated through a series of functions from include/linux/debugfs.h. In non-technical lingo, it is a seperate whiteboard where linux kernel developers can write for their own debugging purposes, and which gets wiped out everytime the computer is restarted :)

If you think this is wrong, or needs more evidence to support the action, please let me know.

For very basic introduction, please read [this small writeup.] Thanks.

Knight Samar (talk) 21:17, 5 May 2010 (UTC)

Htree in ext2

Apparently, using htree in ext2 to get better directory performance has been available for some time (I've been using it for several years now), so the sentence To find a file, the directory is searched front-to-back for the associated filename. For reasonable directory sizes, this is fine. But for huge large directories this is inefficient, and ext3 offers a second way of storing directories that is more efficient than just a list of filenames. is wrong in my eyes.

I've had trouble finding authoritative sources on this (apart form my own experience), since this misconception appears to be widely spread (including here at Wikipedia). --Din Ycae (talk) 07:57, 11 January 2011 (UTC)

Never mind, I was wrong. The filesystem can be created by the ext2 tools with the proper bits in place (namely, the possibility of using Htree for directories), but the ext2 code makes no use of the feature. — Preceding unsigned comment added by Din Ycae (talk • contribs) 11:08, 11 January 2011 (UTC)

External links modified

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Clarify time when ext2 was replaced by ext3

It was around when Debian Potato was released (see https://wiki.debian.org/Ext3) but I'm not great at writing and don't want to mess anything up. — Preceding unsigned comment added by Dogesoft (talk • contribs) 03:16, 13 July 2020 (UTC)