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Filesystems

A large number of changes have been made to the still experimental Btrfs filesystem. Among them is the newly developed "Delayed Inode Items Operation", which according to the commit comment improves performance by about 15 per cent when creating files, and by about 20 per cent when deleting them. Another addition is scrub support, which allows users to instruct the kernel to read all data from the storage medium and run through all the checksums to ensure data integrity. As Btrfs uses Copy on Write (COW), it can quickly fragment with certain access patterns; the new "auto_defrag" mount option instructs the filesystem to detect "small random writes" in existing files and queue the files for automatic defragmentation.

Ext4 now supports the "punch hole" function that is accessible via fallocate and allows programs to inform the filesystem of file areas that don't contain any data. This feature can make a difference for virtualisation programs, because it enables the filesystem to deallocate disused memory areas in sparse file hard disk images. The XFS filesystem now supports online discard, which allows storage media to be informed of newly deallocated memory areas directly when deleting files – this is interesting for SSDs and network storage solutions that use thin provisioning.

Drivers

  • It wasn't until the sixth release candidate that the kernel hackers made the greatest change in the storage area: the isci driver for the SAS controller in Intel's C600 server and workstation chipset, which is to be released in the next few months (see 1).
  • The XHCI driver for USB 3.0 reportedly now supports the EHCI/XHCI Port switching function in Panther Point chipsets, which are part of the Ivy Bridge processors that Intel will probably launch at the beginning of next year (1, 2).
  • The k10temp driver in Linux 3.0 can address AMD's 15H (code name "Bulldozer") processor family, which will probably go on sale in late summer or autumn. The new driver fam15h_power provides interfaces for the provision of information about the power consumption of Bulldozer CPUs.
  • The media subsystem now offers basic functions for DVB-T2 hardware (1, 2) and includes a driver for Sony's CXD2820R DVB-T2 chip.
  • The media subsystem includes a driver for the Micronas DRXD DVB-T tuner used by various vendors (see 1, 2). A basic driver for Microsoft's Kinect has also been added.
  • The kernel's audio drivers developed as part of the Alsa now include one for the Digigram Lola PCIe soundcard and another for Apple's iSight microphone.

Architecture and Infrastructure

First efforts have been made to clean up the ARM code after Linus Torvalds had, in the past few months, criticised rather sharply the code quality and the work practices of the ARM code developers. Various changes were made to the code for rebooting PCs; through these modifications, the way Linux reboots is now similar to that of recent Windows versions (for example 1). The measures are designed to fix rebooting issues on a variety of systems, including Apple systems and various Thinkpad notebooks.

The kernel hackers have removed the prefetching feature in various kernel areas (for example 1, 2, 3); the feature provides the capability to explicitly request data just before the data is used. Although this should improve performance because the data is moved into the CPU cache before it is used, it has recently been found that, in certain situations, many modern processors can do this so well themselves that kernel prefetching can even have a negative effect on performance. Details can be found in the article "The problem with prefetch" on LWN.net.

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