8 On Sun, Dec 31, 2023, 5:26 PM Grant Taylor via TUHS wrote: > On 12/31/23 11:38, arnold@skeeve.com wrote: > > The different overlapping partitions predates disk labels. > > Okay. That in and of itself doesn't surprise me much that a convention > of overlapping partitions was carried forward from the driver based > partitioning into label based partitioning. > Many editors dont let you do this... > Up to and including 4.3 BSD, to change the size of partitions on a > > particular disk, you had to recompile the kernel. > > So I've learned over the last couple of years as I read more about Unix > history. > > > They were that way so that if you had multiple disks, you could use > > one for root + swap + some thing small and use another whole disk > > for a single filesystem. > > I'm not understanding how /overlapping/ partitions helps make use of > portions of disks. > Maybe I should back up and ask for clarification. What /overlapping/ > partitions means in this context? > It means you either use one set of non overlapping partitions or another set. They were setup in clever ways My naive assumption was that partition -- I use that term loosely -- "c" > overlaps / contains / all other partitions on the disk; "a", "b", and > maybe "d". > > I'm eliding the "c" MBR partition vs "d" entire drive" distinction for > the moment. > > I see some value in the "c" partition being the entire disk as used by > BSD so that it's possible to point backup / restore / copy utilities at > the entire disk. > > But I don't understand value in having partitions overlap / contain each > other's blocks, save for backup via "c". > > I do see some value in extending the "c" is the entire MBR partition > methodology to "d" is the entire disk containing multiple MBR > partitions. Again, the value seems to be in backup and recovery. > > But I still simply do not understand why I would ever want partition "e" > to be blocks 100-199, partition "f" to be blocks 195-299, and partition > "g" to be blocks 295-399. What value is there in having partitions e, > f, and g overlap each other? > > I get dd if=/dev/0c of=/dev/rmt. Or even /dev/0d. > > I fail to understand why I'd ever want other partitions to overlap. > It's more like you can use two or three partitions with non overlapping sets that cover the whole disk. > It was also helpful, if you had the drives, to nightly dd your real > > root to the "a" partition on another, identical drive, so that you > > could boot the backup root in an emergency. > > Sure. But I don't see what that has to do with overlapping partitions. > > I'd naively think that I could do something like the following: > > dd if=/dev/0a of=/dev/1a > > And get the same effect. > > > I am guessing that the original conventions date back to V7 or 32V, > > but one would have to go looking at code to be sure. > > "a" for root makes some intuitive sense as the root file system is > required to do anything else. > > Then when you want swap, the next partition is "b". > > Wanting another partition that is the entire disk (as seen by BSD) makes > some logical sense to me too, so "c". > > Were subsequent partitions sort of used as needed and had less > consistency? Especially when "d" because the entire disk containing > multiple MBR partitions when "c" was restricted to the MBR partition the > label was in? > > Aside: > > Would that mean that the following "d" partitions would be the same > thing, as in the entire /dev/ad0 disk? > > /dev/ad0s0d > /dev/ad0s1d > > Wherein I'm borrowing the FreeBSD slice nomenclature -- as I understand > it -- to identify the first (zero) and second (one) MBR partition on > /dev/ad0 > Yes. But ancient Unix didn’t have nested partitioning schemes like FreeBSD supports... History and how we got to where we are today can be both very confusing > and even more enlightening once you understand it. What's more is that > once you understand it, things start making more intuitive sense when > you look at them. > Think more of a limited number of ways to mix and match for greater flexibility w/o editing the tables. A silly example: a is first 2/3 of the disk. B is 2nd 2/3, c d and e are 1/3 each. Warner -- > Grant. . . . >