Hitchhiker's Guide to Openbsd

14.6 - Soft Updates
Soft Updates is based on an idea proposed by 
Greg Ganger and Yale Patt
 and developed for FreeBSD by 
Kirk 
McKusick
. SoftUpdates imposes a partial ordering on the buffer cache operations which permits the requirement 
for synchronous writing of directory entries to be removed from the FFS code. Thus, a large performance increase 
is seen in disk writing performance. 
Enabling soft updates must be done with a mount-time option. When mounting a partition with the 
mount(8)
utility, you can specify that you wish to have soft updates enabled on that partition. Below is a sample 
/etc/fstab
(5)
entry that has one partition sd0a that we wish to have mounted with soft updates. 
/dev/sd0a / ffs rw,softdep 1 1
Note to sparc users: Do not enable soft updates on sun4 or sun4c machines. These architectures support only a 
very limited amount of kernel memory and cannot use this feature. However, sun4m machines are fine. 
14.7 - How do OpenBSD/i386 and OpenBSD/amd64 boot?
The boot process for OpenBSD/i386 and OpenBSD/amd64 is not trivial, and understanding how it works can be 
useful to troubleshoot a problem when things don't work. There are four key pieces to the boot process: 
1. Master Boot Record (MBR): The Master Boot Record is the first 512 bytes on the disk. It contains the 
http://www.openbsd.org/faq/faq14.html (12 of 34)9/4/2011 10:02:25 AM

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14 - Disk Setup
primary partition table and a small program to load the Partition Boot Record (PBR). Note that in some 
environments, the term "MBR" is used to refer to only the code portion of this first block on the disk, 
rather than the whole first block (including the partition table). It is critical to understand the meaning of 
"initialize the MBR" -- in the terminology of OpenBSD, it would involve rewriting the entire MBR, 
clearing any existing partition table, not just the code, as it might on some systems. You will often not 
want to do this. Instead, use fdisk(8)'s "-u" command line option ("
fdisk -u wd0
") to (re)install the 
MBR boot code. 
While OpenBSD includes its own MBR code, you are not obliged to use it, as virtually any MBR code can 
boot OpenBSD. The MBR is manipulated by the fdisk(8) program, which is used both to edit the partition 
table, and also to install the MBR code on the disk. 
OpenBSD's MBR announces itself with the message: 
Using drive 0, partition 3.
showing the disk and partition it is about to load the PBR from. In addition to the obvious, it also shows a 
trailing period ("."), which indicates this machine is capable of using LBA translation to boot. If the 
machine were incapable of using LBA translation, the above period would have have been replaced with a 
semicolon (";"), indicating CHS translation: 
Using drive 0, partition 3;
Note that the trailing period or semicolon can be used as an indicator of the "new" OpenBSD MBR, 
introduced with OpenBSD 3.5. 
2.