"15 partitions (at least for sd_mod devices) are too few," Jan Engelhardt suggested along with a patch to try and make the mounting of an unlimited number of partitions possible. H. Peter Anvin proposed as an alternative, "now when we have 20-bit minors, can't we simply recycle some of the higher bits for additional partitions, across the board? 63 partitions seem to have been sufficient; at least I haven't heard anyone complain about that for 15 years."
Alan Cox explained, "this was proposed ages ago. Al Viro vetoed sparse minors and it has been stuck this way ever since. If you have > 15 partitions use device mapper for it. I'd prefer it fixed but it's arguable that device mapper is the right way to punt all our partitioning to userspace".
"This feature allows a read-only view into a read-write filesystem. In the process of doing that, it also provides infrastructure for keeping track of the number of writers to any given mount," Dave Hansen began, describing his "read-only bind mounts" patches. He continued, "this has a number of uses. It allows chroots to have parts of filesystems writable. It will be useful for containers in the future because users may have root inside a container, but should not be allowed to write to some filesystems. This also replaces patches that vserver has had out of the tree for several years. It allows security enhancements by making sure that parts of your filesystem [are] read-only (such as when you don't trust your FTP server), when you don't want to have entire new filesystems mounted, or when you want atime selectively updated."
Christoph Hellwig was interested in seeing the patches get some more testing, "I still think we really want this in -mm. As we've seen at the kernel summit there's a pretty desperate need for it." Andrew Morton noted that the "unprivileged mounts" code was working in the same area, but described that work as "a bit stuck." He suggested, "it sounds like a better approach would be for me to merge the r/o bind mounts code and to drop (or maybe rework) the unprivileged mounts patches." Dave explained that they don't collide much, to which Andrew's reply suggested that the read-only mount patches would be merged into the -mm kernel soon.
In a recent lkml thread, Linus Torvalds was involved in a discussion about mounting filesystems with the
noatime option for better performance, "'noatime,data=writeback' will quite likely be *quite* noticeable (with different effects for different loads), but almost nobody actually runs that way." He noted that he set O_NOATIME when writing git, "and it was an absolutely huge time-saver for the case of not having 'noatime' in the mount options. Certainly more than your estimated 10% under some loads." The discussion then looked at using the
relatime mount option to improve the situation, "relative atime only updates the atime if the previous atime is older than the mtime or ctime. Like noatime, but useful for applications like mutt that need to know when a file has been read since it was last modified." Ingo Molnar stressed the significance of fixing this performance issue, "I cannot over-emphasize how much of a deal it is in practice. Atime updates are by far the biggest IO performance deficiency that Linux has today. Getting rid of atime updates would give us more everyday Linux performance than all the pagecache speedups of the past 10 years, _combined_." He submitted some patches to improve
relatime, and noted about
"It's also perhaps the most stupid Unix design idea of all times. Unix is really nice and well done, but think about this a bit: 'For every file that is read from the disk, lets do a ... write to the disk! And, for every file that is already cached and which we read from the cache ... do a write to the disk!'"