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Ext4 (1).pdf - Basharat Anis Extra Credit Paper: EXT4 File...

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Basharat AnisExtra Credit Paper: EXT4 File SystemAbstract:Ext4 file system is the ext3 file system successor. It is supported on today’smost popular Linux distributions (RedHat, Ubuntu, Fedora). In contrastto the 32-bit ext3 file system, that has only some features added to its predecessor ext2 andmaintains a data structure as in the ext2 file system, the ext4 file system has integrated moresubstantial changes compared to ext3. Ext4 has improved data structure and enhancedfeatures, which brought more reliability and efficiency. It is a 64-bit, allowing the file size ofup to 16 TB. Great efforts that have been put into the process of ext4 development resulted innew features and techniques: extents, journaling check summing, simultaneous allocation ofmultiple units, delayed allocation, faster fsck (file system check), and online defragmentationof small and large size directories. This way formed folders can have up to 64,000 files.The ext4 filesystem can support volumes with sizes up to 1exbibyte (EiB)and single fileswith sizes up to 16tebibytes (TiB)with the standard 4KiB block size.[12]The maximum file,directory, and filesystem size limits grow at least proportionately with the filesystem blocksize up to the maximum 64 KiB block size available onARMandPowerPC/Power ISACPUs.ext4 isbackward-compatiblewithext3andext2, making it possible tomountext3 and ext2as ext4. This will slightly improve performance, because certain new features of the ext4implementation can also be used with ext3 and ext2, such as the new block allocationalgorithm, without affecting the on-disk format.ext4 can pre-allocate on-disk space for a file. To do this on most file systems, zeroes wouldbe written to the file when created. In ext4 (and some other files systems such asXFS)fallocate(), a new system call in the Linux kernel, can be used. The allocated spacewould be guaranteed and likely contiguous. This situation has applications for mediastreaming and databases.ext4 useschecksumsin the journal to improve reliability, since the journal is one of the mostused files of the disk. This feature has a side benefit: it can safely avoid a disk I/O wait duringjournaling, improving performance slightly. Journal checksumming was inspired by aresearch article from theUniversity of Wisconsin, titledIRON File Systems(specifically,section 6, called "transaction checksums"), with modifications within the implementation ofcompound transactions performed by the IRON file systemEXT4 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 the
addition of intervals down to nanoseconds. The addition of two high-order bits in thetimestamp field defers theYear 2038 problemuntil 2446—for EXT4 filesystems, atleast.

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