Parallel installations, but it seems it was easier to implement It would be possible to fix such packages to support Packages are not so enlightened, allowing only one version to be installedĪt a time. ThisĪllows multiple versions to be installed side by side. Support files, such as /usr/lib/perl5/site_perl/5.22.1. Many packages, such as Perl andĮmacs, include a version number in the path names used for finding The example he sketched was to support multiple versions of packages In a container, the process shouldn't be able to get out. Has been the creation of containers to contain processes - once The situation with mount namespaces brings its own set of problems,
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Sufficient to justify a rewrite of the symlink lookup code - is tinyĬompared to the cost of scheduling a user-space process to provide an Minnaar identified compilation as a problematic case forįUSE-based variant symlinks too, and even the cost of that spinlock. This was found to be particularly true whenĬompiling code, since searching for include files generates lots ofįilename lookups. Nature, a variant symbolic link cannot be cached in the VFS layer, soĮvery request would need to go to user space and back into theįor some workloads, even requiring spinlocks for following a symlinkĬan be too expensive. Single symbolic link, a performance decrease can still be measured. Is not used for any filesystem I/O and is only needed to look up a The problem with a FUSE-based solution is performance.
Those ideas had been tried and found wanting. History, but was something he has been working on for some time. Minnaar made it clear that this wasn't just a new idea with no Mount namespaces to give different processes a different view of the Minnaar was told that he should use a FUSE filesystem The responses to this proposal were pretty much as would be expected: # mount -t varsymfs -o UNIVERSE none /.universe To use this you would mount a filesystem at some well known locationĪnd create links that pass though that location. Reports the value of that variable as the content of the symlink. When any process reads or follows this link, theįilesystem looks though that process's environment for a particularĮnvironment variable, specified when the filesystem is mounted, and Theįilesystem provides a single directory that contains a single symlinkĬalled resolve. Module - takes an extremely simple approach to the problem. Still pops up from time to time, most recently in a proposal by Cole Minnaarįor a "Variant Symlink Filesystem". While those issues are long behind us, the desire for variant symlinks See /bin as a symlink to /.ucbbin, while others This would allow, for example, some processes to University of California at Berkeley), and System V Unix from AT&T.ĭetails varied, but the core idea was that some attribute of a processĬould be used to modify the target of a symlink or to select among Unix systems wanted to be compatible with both BSD Unix from UCB (The They haveĪ history going back at least to the 1980s when various vendors of On details of the process that reads or follows the link. Variant symlinks are symbolic links that behave differently depending This article was contributed by Neil Brown
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