"Another week, another -rc," began Linus Torvalds, announcing the 2.6.27-rc4 Linux kernel, continuing, "this time the diffstat is almost totally dominated by the addition of the musb driver that drives the MUSB and TUSB controllers integrated into omap2430 and davinci. That, together with the removal of the auerswald USB driver (replaced by libusb version) is more than half of the bulk of the patch, and obviously most users won't ever notice." Linus added:

"Apart from those bulky USB updates, there's some arch updates (blackfin and ia64), network and input driver updates, and an XFS and UBIFS update. The rest is mostly random stuff all over, probably best described by the appended shortlog. A number of regressions should be off the table, but more remain..."

"I'd like to get a first round of review on my AXFS filesystem," began Jared Hulbert, describing his new Advanced XIP File System for Linux. XIP stands for eXecute-In-Place. The new filesystem received quite a bit of positive feedback. Jared offered the following description:

"This is a simple read only compressed filesystem like Squashfs and cramfs. AXFS is special because it also allows for execute-in-place of your applications. It is a major improvement over the cramfs XIP patches that have been floating around for ages. The biggest improvement is in the way AXFS allows for each page to be XIP or not. First, a user collects information about which pages are accessed on a compressed image for each mmap()ed region from /proc/axfs/volume0. That 'profile' is used as an input to the image builder. The resulting image has only the relevant pages uncompressed and XIP. The result is smaller memory sizes and faster launches."

"The latest feature release GIT 1.6.0 is available at the usual places," began Git maintainer, Junio Hamano, announcing the latest stable release of the distributed version control system originally written by Linus Torvalds. Among the current changes, Junio noted, "with the default Makefile settings, most of the programs are now installed outside your $PATH, except for 'git', 'gitk' and some server side programs that need to be accessible for technical reasons." He continued, "by default, packfiles created with this version uses delta-base-offset
encoding introduced in v1.4.4. Pack idx files are using version 2 that allows larger packs and added robustness thanks to its CRC checking, introduced in v1.5.2 and v1.4.4.5." Julio highlighted several other changes, including the addition of a '.sample' extension to the default trigger scripts to be sure they don't execute in a default install, and the removal of the 'stupid' merge strategy. Other changes include:

"Git-gui learned to stage changes per-line; Reduced excessive inlining to shrink size of the 'git' binary; When an object is corrupt in a pack, the object became unusable even when the same object is available in a loose form, we now try harder to fall back to these redundant objects when able; performance of 'git-blame -C -C' operation is vastly improved; even more documentation pages are now accessible via 'man' and 'git help'; longstanding latency issue with bash completion script has been addressed; pager. configuration variable can be used to enable/disable the default paging behaviour per command; git-cvsserver learned to respond to 'cvs co -c'; 'git-diff -p' learned to grab a better hunk header lines in BibTex, Pascal/Delphi, and Ruby files and also pays attention to chapter and part boundary in TeX documents; error codes from gitweb are made more descriptive where possible, rather than '403 forbidden' as we used to issue everywhere; git-merge has been reimplemented in C."

A recent discussion on the Linux Kernel mailing list noted that threaded 64-bit applications suffer a drastic slowdown in pthread_create performance when stack utilization goes above 4GB. Ingo Molnar offered an explanation of the problem, "unfortunately MAP_32BIT use in 64-bit apps for stacks was apparently created without foresight about what would happen in the MM when thread stacks exhaust 4GB. The problem is that MAP_32BIT is used both as a performance hack for 64-bit apps and as an ABI compat mechanism for 32-bit apps. So we cannot just start disregarding MAP_32BIT in the kernel - we'd break 32-bit compat apps and/or compat 32-bit libraries." The original report noted that once the shared stack goes above 4GB in size, thread creation can take as long as 10 milliseconds, a slowdown described as "quite unacceptable".

Ingo created a patch introducing a new MAP_STACK flag for glibc to be used instead of MAP_32BIT and avoid imposing the 32-bit performance limitation on threaded 64-bit applications. He noted, "glibc can switch to this new flag straight away - it will be ignored by the kernel." The new flag was quickly merged upstream, and changes were planned for glibc.

"It is about time to take a step back and describe what I have been implementing," began Daniel Phillips, referring to his new Tux3 filesystem. He provided a simple ASCII diagram that detailed the filesystem's hierarchical structure, describing each of the elements. About one he noted, "the volume table is a new addition not central to the goals of Tux3, but a nice feature to have given that it comes nearly for free. One Tux3 volume can have an arbitrary number of separate filesystems tucked inside it, indexed by a simple integer parameter at mount time. People say they like this idea and it imposes no significant complexity, so it goes in." Daniel continued:

"Each volume has a metablock pointing at the forward log chain for the volume, a version table that describes the hierarchical relationship between versions (snapshots), an atime table to take care of that horrid legacy Unix feature, and an inode table containing files and attributes of files. [...] Versioning takes place in three places, versioned pointers in the atime btree, versioned extents in a file data btree and versioned attributes in the inode table. [...] Notice the absence of a journal, the functionality of which is provided by forward log elements that I described in the Hammer thread (and will eventually write a separate post about)."

"Things really _have_ calmed down, and hopefully we've also resolved a lot of the regressions in -rc3," began Linus Torvalds, announcing the 2.6.27-rc3 Linux kernel. He noted that much of the patch size was from the inclusion of the new ath9k wireless driver, with much of the rest of the patch size due to the renaming of many arch include files in the ARM, AVR32 and m68lnommu architectures. Linus continued:

"All the small changes are where the regression fixes are, and other random improvements. And they're all over. The ShortLog (appended) probably gives a taste of it."

Mikulas Patocka announced new patches introducing snapshot merging for the Linux kernel's logical volume manager. He explained, "snapshot merging allows you to merge snapshot content back into the original device. The most useful use for this feature is the possibility to rollback [the] state of the whole computer after [a] failed package upgrade, [or an] administrator's error". The patches are for the 2.6.26 kernel, with device mapper 1.02.27 and LVM2.2.02.39.

Mikulas noted that there are three types of merges supported, --nameorigin, --namesnapshot, and --onactivate. The default merge method is --nameorigin, which can merge a snapshot into the origin volume, which can be mounted at any time after the merge starts. The --namesnapshot method merges into a snapshot, which can then be mounted. And the --onactive method schedules a merge to happen the next time the volume is activated, such as during a reboot. Mikulas noted, "this implementation of snapshot merging is meant to be stable, report any possible bugs to me."