OpenBSD Mastery: Filesystems by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 0000000000000
Published: 2023-02-15T00:34:46+00:00

Growing Filesystems

You add disks to free up space on your existing disks. Sometimes that disk isn’t partitioned the way you want, however. If you combine everything we’ve learned, and are lucky, you can change that without reinstalling. You’ll also need basic math, like subtracting large numbers and correctly multiplying seven by nine.

The growfs(8) program lets you expand a filesystem, but it’s trickier than it sounds. A filesystem can only be expanded into empty space immediately after the existing filesystem. If you have an empty partition at the beginning of the disk, but want to grow a filesystem at the end of the disk, you’re out of luck and all you can do is say, “oh no not again.” But let’s see how this works.

The trick to growing a filesystem is that a filesystem is just a bunch of bits on a partition. OpenBSD partitions are just chalk lines between sectors. When you create a filesystem, it grows until it hits the line and stop. By erasing some of those lines, you create a larger partition. You use growfs(8) to tell the filesystem “hey, you have more room.”

One of my hosts has a 1.6GB /usr/local partition. When I installed the host, I thought it was big enough. I was wrong. Let’s see what might be done. Partitions are defined in the disklabel, so let’s look.

…

# size offset fstype [fsize bsize cpg]

…

g: 1118240 9041472 4.2BSD 2048 16384 8666 # /usr/X11R6

h: 3756224 10159712 4.2BSD 2048 16384 12960 # /usr/local

i: 960 64 MSDOS

j: 3293184 13915936 4.2BSD 2048 16384 12960 # /usr/src

k: 10928160 17209120 4.2BSD 2048 16384 12960 # /usr/obj

l: 5415520 28137280 4.2BSD 2048 16384 12960 # /home

Partition h is /usr/local. It is immediately followed by partition i, the EFI boot partition. You can manipulate disks in many ways, but eliminating the boot loader is frequently counterproductive. But look more closely. Partitions are shown in letter order, not block order. All those horrid size and offset numbers tell the real story. Partition i begins on sector 64 and has an offset of 64 sectors. The only thing in front of it is the actual GUID partition table.

Partition h has a size of 3756224 sectors and an offset of 10159712. The partition that really follows it will have an offset of (3,756,224 + 10,159,712 = ) 13,915,936. That’s the offset of partition j, or /usr/src. Adding the size and offset of partition j gives us (3,293,184 + 13,915,936 = ) 17,209,120, the beginning of partition k or /usr/obj.

I perfectly well understand why OpenBSD creates these partitions. It’s faster to newfs(8) /usr/obj than recursively delete files, and adding the partitions after install is impossible. But this host uses binary updates via sysupgrade(8), and I have many copies of the OpenBSD source tree at hand. These partitions can go away.

Before editing filesystems or partitions on a live system, perform a complete backup. Schedule the work for a time that you could perform a complete reinstall and restore. A basic math error can wreck the system.

We could do this in single-user mode, but I believe I’m an advanced life form and see no reason to bother.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.