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SquashFS, UDF, Fallocate and NFS

Used as a file system in a number of Linux distributions for live and installation media, SquashFS in 2.6.38 supports the compression of file system images with the XZ compression format spun out of LZMA (1, 2). Because it is known for very great compression density, some live media will probably be able to offer a few more programs than before. Fedora developers want to use it in version 15, expected in May, and say that a live DVD with games is around 8 per cent smaller when compressed with XZ. For some distributions that already had the predecessor of the newly integrated patches, the new compression method is old hat, although Torvalds did not want to include them in 2.6.34.

Used with optical media (such as DVDs), the UDF file system does without the Big Kernel Lock (BKL) starting in 2.6.38. The developers have therefore removed BKL from the last widely used kernel component that still required BKL, following the BKL removal campaign in 2.6.37. There is also a rough roadmap to rid the world of the remaining users of BKL.

The Fallocate system call can be used by programs to provide the file system with information about unused areas in the middle of files (1, 2; LWN: "Punching holes in files"). For now only supported by OCFS2 and XFS, the mechanism reportedly allows, say, virtualisation programs to release no longer used memory areas on hard drive images available as sparse files; to do so, however, the guest system somehow has to let the host know which memory areas are no longer used.

The VFS now provides improved support for automount as explained in a patch review (see 1, 2, 3). In his main git pull request, NFSd4 maintainer J. Bruce Fields emphasises a number of the improvements to the NFS4 daemon required for server-side support of NFS 4.1; in doing so, he refers to a page in the NFS wiki that provides an overview of the remaining changes needed for 4.1 support.

Minor gems

Many further minor, but by no means insignificant, changes can be found in the list below, which contains the commit headers referring to the respective change. Like many of the references in the text above, the links point to the relevant commit in the web front end of the Git branch for the "official" kernel sources maintained by Linus Torvalds at The commit comments available at these links and the patches themselves provide extensive further information on the respective changes.

Every link is preceded by various letters and numbers in square brackets. The letter "C" identifies patches that modify Kconfig files, which contain the help texts and configuration options displayed by "make menuconfig", "make xconfig" and similar tools during kernel configuration. "D" is used for patches that modify the documentation available under Documentation/ in the kernel branch. "N" identifies changes which create a new file. The numbers provide a rough idea of the patch size. For example, "1" is used for changes between 10 and 20 KBytes including comment, "2" for patches between 20 and 30 KBytes; changes without a number are less than 10 KBytes, while patches marked "9" are 90 KBytes or more.

For other articles on 2.6.38 and links to the rest of the "Coming in 2.6.38" series, see The H's Kernel Log - 2.6.38 Tracking page. New editions of Kernel Logs are also mentioned on and Twitter via "@kernellog2". The Kernel Log author also posts updates about various topics on and Twitter via "@kernellogauthor".

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