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ext4.docx - EXT4 File System: The EXT4 filesystem primarily...

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EXT4 File System:TheEXT4 filesystemprimarily improves performance, reliability, and capacity. Toimprove reliability, metadata and journal checksums were added. To meet variousmission-critical requirements, the filesystem timestamps were improved with theaddition of intervals down to nanoseconds. The addition of two high-order bits in thetimestamp field defers theYear 2038 problemuntil 2446—for EXT4 filesystems, atleast.In EXT4, data allocation was changed from fixed blocks to extents. An extent isdescribed by its starting and ending place on the hard drive. This makes it possible todescribe very long, physically contiguous files in a single inode pointer entry, whichcan significantly reduce the number of pointers required to describe the location of allthe data in larger files. Other allocation strategies have been implemented in EXT4 tofurther reduce fragmentation.EXT4 reduces fragmentation by scattering newly created files across the disk so thatthey are not bunched up in one location at the beginning of the disk, as many early PCfilesystems did. The file-allocation algorithms attempt to spread the files as evenly aspossible among the cylinder groups and, when fragmentation is necessary, to keep thediscontinuous file extents as close as possible to others in the same file to minimizehead seek and rotational latency as much as possible. Additional strategies are used topre-allocate extra disk space when a new file is created or when an existing file isextended. This helps to ensure that extending the file will not automatically result inits becoming fragmented. New files are never allocated immediately after existingfiles, which also prevents fragmentation of the existing files.Aside from the actual location of the data on the disk, EXT4 uses functionalstrategies, such as delayed allocation, to allow the filesystem to collect all the databeing written to the disk before allocating space to it. This can improve the likelihoodthat the data space will be contiguous.Older EXT filesystems, such as EXT2 and EXT3, can be mounted as EXT4 to makesome minor performance gains. Unfortunately, this requires turning off some of theimportant new features of EXT4, so I recommend against this.EXT4 FileSystem Structure :Aninodeis a data structure on a filesystem on Linux and other Unix-like operatingsystems that store all the information about a file except its name and its actual data.When a file is created, both the file names and their corresponding inode numbers arestored as entries in the directory that appears to the user to contain the files. That isthe directory associates file names with inodes.Aninode number, which is an integer that is unique within the filesystem.
The operating system obtains a file’s inode number and information in the inodethrough the use of the system call namedstat.

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