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| H A D | open.c | diff 269308 Wed Jul 30 16:08:16 MDT 2014 marcel Provide a means for loaders to control which file system to use. This to counteract the default behaviour of always trying each and every file system until one succeeds, or the open fails. The problem with the loader is that we've implemented features based on this behavior. The handling of compressed files is a good example of this. However, it is in general highly undesirable to not have a one-time probe (or taste in the geom lingo), followed by something similar to a mount whenever we (first) read from a device. Everytime we go to the same device, we can reasonably assume it (still) has the same file system. For file systems that need to do far more that a trivial read of a super block, not having something similar to a mount operation is disastrous from a performance (and thus usability) perspective. But, again, since we've implemented features based on this stateless approach, things can get complicated quickly if and when we want to change this. And yet, we sometimes do need stateful behaviour. For this reason, this change simply introduces exclusive_file_system. When set to the fsops of the file system to use, the open call will only try this file system. Setting it to NULL restores the default behaviour. It's a low-cost (low-brow?) approach to provide enough control without re-implementing the guts of the loader. A good example of when this is useful is when we're trying to load files out of a container (say, a software packaga) that itself lives on a file system or is fetched over the network. While opening the container can be done in the normal stateless manner, once it is opened, subsequent opens should only consider the container. Obtained from: Juniper Networks, Inc. |
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