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Device Mapper and MD

New interfaces enable the Device Mapper to access the MD (Multiple Devices) subsystem's software RAID admin functions for RAID levels 4, 5 and 6. Together with a fresh version of DMRAID this should, in the long term, improve the kernel's support for the host RAID 5 features offered by many motherboard chip-sets. These features were previously only accessible via special kernel patches that aren't included in all distributions.

The use of multiple work queues in Dmcrypt is designed to improve the Device Mapper's (DM) handling of multi-core processors and should speed up encryption and decryption. Also newly added are the multi-key capabilities for Dmcrypt that allow individual blocks on a storage device to be encrypted with a separate key; this is the basis of the newly supported loop-AES block device encryption system. From 2.6.38, the DM's mirroring / RAID1 code will pass on discard information to inform storage media about newly available areas. Two changes submitted by Mikulas Patocka improve the Device Mapper's performance under certain conditions, as demonstrated by measurements he included in the commit comment (1, 2).

In brief

  • The Blkio cgroups controller for controlling storage utilisation now supports a basic form of hierarchical cgroups. Further details can be found in the article "Hierarchical group I/O scheduling" on that describes a patch set, still under development, that removes some of the limitations of the solution merged for 2.6.38.
  • In his main Git-Pull request, block subsystem maintainer Jens Axboe highlights various performance optimisations made to the CFQ (Completely Fair Queuing) I/O scheduler (such as 1, 2).
  • The code for Multiple Devices (MD) now offers RAID 1 to RAID 0 take-over support.
  • In the 2.6.38 merge window, the developers integrated a patch to make the block layer check whether the subsystems that are based on it observe the read-only flag. However, this caused various problems, as explained in an article on that will be freely available from 3 March, and the developers have, for the time being, removed the code.
  • From 2.6.38, the megaraid_sas driver will address the 9265 and 9285 MegaRAID chips.
  • A driver for the Acard ATP8620 has been added to the Libata subsystem.
  • The lpfc (LightPulse Fibre Channel) SCSI driver for Emulex chips now supports SLI4 FC discovery.

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.

Block-Layer, Libata, Infiniband


MMC, MFD, MTD and Co.

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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