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Storage

From 3.2, device-mapper contains an experimental persistent data library, a framework for storing metadata for device mapper targets. One of the first device-mapper targets to use the new infrastructure is dm-thin, which enables functions for thin provisioning of storage, allowing more storage to be exported than is actually present. Dm-thin also brings improved snapshot functionality, which makes more efficient use of storage space.

Infrastructure

The writeback code now throttles programs which generate large volumes of data for writing to storage media more efficiently (e.g. 1, 2, 3, 4, 5). This should ensure that the system reacts to user interaction quickly and does not become overloaded by trying to cache large volumes of data, such as can occur with dd when writing to a very slow disk. In the release email for 3.2-rc1, Torvalds noted that, although these changes were very small, they had the potential to be noticed by all users. There are, however, still some situations in which, because the memory management is busy, the system appears to the user to be reacting slowly. Mel Gorman has developed changes (e.g. 1, 2), to fix a frequent problem relating to transparent huge pages (THP), background information on which can be found in this LWN.net article.

With the help of the new CFS bandwidth controller and the control groups (cgroups) infrastructure, it is now possible to cap process group CPU time. One use case of this is to permit cloud operators to ensure that users are not able to consume more CPU resources than they have paid for (e.g. 1, documentation). The kernel now tries to mark itself as TAINT_OOT_MODULE (tainted: out of tree module) when it loads modules which are not part of the official Linux kernel. This will enable developers looking at bug reports to quickly recognise that they are dealing with such an environment. Out of tree modules have a bad reputation in kernel circles and can occasionally cause problems in areas which at first glance appear to have little to do with them.

In combination with a TPM, the extended verification module (EVM) (merged in Linux 3.2) is now able to detect modifications to system files, thus ensuring that malicious code is not loaded on booting. The clock source driver for Microsoft's Hyper-V virtualisation technology has moved from the staging area to the x86 architecture code. With Microsoft programmers having made numerous improvements, various other drivers for core Hyper-V technologies have also managed to leave the staging directory. The Hyper-V drivers for emulated mouse, network and storage hardware will remain in the staging area for the time being, but will probably move for 3.3.

Architecture

Various patches (e.g. 1) allow Linux 3.2 to avoid the effects of an L1 instruction cache peculiarity in AMD's Bulldozer processors that can cause performance losses in certain situations. These patches have also been submitted for the series 3.0 and 3.1 stable kernels (1, 2), but haven't yet been integrated there yet.

Linux now supports Qualcomm's Hexagon CPU architecture (e.g. 1, 2). Also newly supported is the upcoming Ivy Bridge platform's random number generator, which can be addressed via the RDRAND x86 instruction (1, 2).

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