Hitchhiker's Guide to Openbsd

14.11 - Mounting disk images in OpenBSD
To mount a disk image (ISO images, disk images created with dd, etc.) in OpenBSD you must configure a 
svnd
(4)
device. For example, if you have an ISO image located at /tmp/ISO.image, you would take the following steps 
to mount the image. 
# vnconfig svnd0 /tmp/ISO.image
# mount -t cd9660 /dev/svnd0c /mnt
Notice that since this is an ISO-9660 image, as used by CDs and DVDs, you must specify type of cd9660 when 
mounting it. This is true, no matter what type, e.g. you must use type ext2fs when mounting Linux disk images. 
To unmount the image use the following commands. 
# umount /mnt
# vnconfig -u svnd0
For more information, refer to the 
vnconfig(8)
 man page. 
14.12 - Help! I'm getting errors with IDE DMA!
DMA IDE transfers, supported by 
pciide(4)
are unreliable with many combinations of older hardware. 
OpenBSD is aggressive and attempts to use the highest DMA Mode it can configure. This will cause corruption of 
data transfers in some configurations because of buggy motherboard chipsets, buggy drives, and/or noise on the 
cables. Luckily, Ultra-DMA modes protect data transfers with a CRC to detect corruption. When the Ultra-DMA 
CRC fails, OpenBSD will print an error message and try the operation again. 
wd2a: aborted command, interface CRC error reading fsbn 64 of 64-
79
(wd2 bn 127; cn 0 tn 2 sn 1), retrying
After failing a couple times, OpenBSD will downgrade to a slower (hopefully more reliable) Ultra-DMA mode. If 
Ultra-DMA mode 0 is hit, then the drive downgrades to PIO mode. 
UDMA errors are often caused by low quality or damaged cables. Cable problems should usually be the first 
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14 - Disk Setup
suspect if you get many DMA errors or unexpectedly low DMA performance. It is also a bad idea to put the CD-
ROM on the same channel with a hard disk. 
If replacing cables does not resolve the problem and OpenBSD does not successfully downgrade, or the process 
causes your machine to lock hard, or causes excessive messages on the console and in the logs, you may wish to 
force the system to use a lower level of DMA or UDMA by default. This can be done by using 
UKC
 or 
config(8)
to change the flags on the 
wd(4)
 device.