Understanding harddisks in openbsd

Disk Partitioning Schemes

OpenBSD:

Differences between Linux and OpenBSD

Things that are different between the OpenBSD and Linux partitioning schemes:

Category Linux OpenBSD
Disk names letters (a, b, c, etc...) numbers (0, 1, 2, etc...)
Partitions on a disk numbers letters
Counting scheme starts counting from "1" starts counting from "0"
Example Linux OpenBSD
First disk /dev/hda /dev/wd0
First partition on second hard disk /dev/hdb1 /dev/wd1a
Counting scheme /dev/hda1 /dev/hda2 /dev/hda3, etc... /dev/wd0a /dev/wd0b /dev/wd0c, etc...

Gain more free space

Abstract

Notes

In the following text it is assumed that the underlying volume (A6 - OpenBSD) was already expanded either by reszing the virtual disk or by changing hardware in the box.

It is also assumed that the harddisk is /dev/sd0.

For simplicity this system has only one big root partition and swap.

Everything is done within OpenBSD's rescue disk kernel bsd.rd. Because no one likes to have separate CD-ROM / USB rescue live utilities ;-)

Start

Boot into the system's rescue disk

Boot bsd.rd by enter this at the kernel boot prompt:

> boot bsd.rd

Then select (S)hell at the prompt.

Re-initialize the MBR, using the entire disk.

# fdisk -i sd0
Do you wish to write new MBR and partition table? [n] y
Writing MBR at offset 0.

Modify the slices

Now we had free space added to the end of a volume.

We have two options now: either resizing an existing slice or adding a new slice.

Option 1: Resize an OpenBSD Partition

Remembering the notes above, this system has only the root partition and swap. The idea is to remove the swap partition, then extend root, and finally put the swap partition back.

Later we need growfs(8) which isn't included with the rescue disk, thats why first we grab it from the existing root partition.

# mount /dev/sd0a /mnt
# cp /mnt/sbin/growfs .
# umount /mnt

Now change the size of the slices:

# disklabel -E sd0
# Inside MBR partition 3: type A6 start 63 size 20964762
Treating sectors 63-20964825 as the OpenBSD portion of the disk.
You can use the 'b' command to change this.

Initial label editor (enter '?' for help at any prompt)
> p
OpenBSD area: 63-20964825; size: 20964762; free: 9446220
#                size           offset  fstype [fsize bsize  cpg]
  a:         10490382               63  4.2BSD   2048 16384    1
  b:          1028160         10490445    swap
  c:         20971520                0  unused
> d
partition to delete: [] b
> p
OpenBSD area: 63-20964825; size: 20964762; free: 10474380
#                size           offset  fstype [fsize bsize  cpg]
  a:         10490382               63  4.2BSD   2048 16384    1
  c:         20971520                0  unused
> c
partition to change size: [] a
Partition a is currently 10490382 sectors in size, and can have a maximum
size of 20964762 sectors.
size: [10490382] 9G
Rounding to cylinder: 18876312
> a b
offset: [18876375]
size: [2088450]
FS type: [swap]
> p
OpenBSD area: 63-20964825; size: 20964762; free: 0
#                size           offset  fstype [fsize bsize  cpg]
  a:         18876312               63  4.2BSD   2048 16384    1
  b:          2088450         18876375    swap
  c:         20971520                0  unused
> w
> q
No label changes.

Create the filesystem

Grow the filesystem using growfs(8):

# growfs /dev/sd0a
We strongly recommend you to make a backup before growing the Filesystem

 Did you backup your data (Yes/No) ? Yes
new file systemsize is: 4719078 frags
Warning: 216792 sector(s) cannot be allocated.
growfs: 9111.1MB (18659520 sectors) block size 16384, fragment size 2048
        using 45 cylinder groups of 202.47MB, 12958 blks, 25984 inodes.
super-block backups (for fsck -b #) at:
 10781088, 11195744, 11610400, 12025056, 12439712, 12854368, 13269024, 13683680, 14098336, 14512992, 14927648, 15342304, 15756960, 16171616, 16586272, 17000928, 17415584, 17830240,
 18244896

...and do a last check:

# fsck /dev/sd0a
** /dev/rsd0a
** Last Mounted on /
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
100205 files, 418854 used, 4172217 free (97 frags, 521515 blocks, 0.0% fragmentation)

MARK FILE SYSTEM CLEAN? [Fyn?] y


***** FILE SYSTEM WAS MODIFIED *****

Option 2: Add a New Partition to OpenBSD

Remembering the notes above, this system has only the root partition and swap. Now we create the new slice as the "d" slice and then put /home onto it.

Add a new slice for /home (the "d" slice)

# disklabel -E sd0
Label editor (enter '?' for help at any prompt)
> p
OpenBSD area: 64-251658225; size: 251658161; free: 209728575
#                size           offset  fstype [fsize bsize  cpg]
  a:         38812976               64  4.2BSD   2048 16384 38128
  b:          3116610         38813040    swap
  c:        251658240                0  unused
> a
partition: [d]
offset: [41929650]
size: [209728575]
FS type: [4.2BSD]
Rounding offset to bsize (32 sectors): 41929664
Rounding size to bsize (32 sectors): 209728544
> p
OpenBSD area: 64-251658225; size: 251658161; free: 31
#                size           offset  fstype [fsize bsize  cpg]
  a:         38812976               64  4.2BSD   2048 16384 38128
  b:          3116610         38813040    swap
  c:        251658240                0  unused
  d:        209728544         41929664  4.2BSD   2048 16384    1
> w
> q
No label changes.

Create the filesystem

# newfs /dev/rsd0d
/dev/rsd0d: 102406.5MB in 209728544 sectors of 512 bytes
506 cylinder groups of 202.47MB, 12958 blocks, 25984 inodes each
super-block backups (for fsck -b #) at:
 32, 414688, 829344, 1244000, 1658656, 2073312, 2487968, 2902624, 3317280,
 3731936, 4146592, 4561248, 4975904, 5390560, 5805216, 6219872, 6634528,
 7049184, 7463840, 7878496, 8293152, 8707808, 9122464, 9537120, 9951776,
 ...
 [snip]
 ...
 193229728, 193644384, 194059040, 194473696, 194888352, 195303008, 195717664,
 196132320, 196546976, 196961632, 197376288, 197790944, 198205600, 198620256,
 199034912, 199449568, 199864224, 200278880, 200693536, 201108192, 201522848,
 201937504, 202352160, 202766816, 203181472, 203596128, 204010784, 204425440,
 204840096, 205254752, 205669408, 206084064, 206498720, 206913376, 207328032,
 207742688, 208157344, 208572000, 208986656, 209401312,

Migrate /home

Here, we mount the existing root filesystem (which contains the old /home) and also the new filesystem.

Finally migrate the data from old /home to new space with tar(1) copying everything from one directory to another, preserving everything (permissions, etc...).

# mount /dev/sd0a /mnt
# mount /dev/sd0d /mnt2
# (cd /mnt/home; tar cf - .) | (cd /mnt2; tar xpf -)
# rm -rf /mnt/home
# mkdir /mnt/home

Don't forget to add the new /home to /etc/fstab and mount it on boot:

# echo "/dev/sd0d /home ffs rw,softdep 0 1" >> /mnt/etc/fstab

# umount /mnt*
# reboot