Hitchhiker's Guide to Openbsd

The entire root partition must be within the computer's BIOS (or boot ROM) addressable 
space. 
Some non-i386 users think they are immune to this, however most platforms have some kind of boot ROM 
limitation on disk size. Finding out for sure what the limit is, however, can be difficult. 
This is another good reason to 
partition your hard disk
, rather than using one large partition. 
fsck(8) time and memory requirements
Another consideration with large file systems is the time and memory required to 
fsck(8)
 the file system after a 
crash or power interruption. One should not put a 120G file system on a system with 32M of RAM and expect it 
to successfully fsck(8) after a crash. A rough guideline is the system should have at least 1M of available memory 
for every 1G of disk space to successfully fsck the disk. Swap can be used here, but at a very significant 
performance penalty, so severe that it is usually unacceptable, except in special cases. 
The time required to fsck the drive may become a problem as the file system size expands, but you only have to 
fsck the disk space that is actually allocated to mounted filesystems. This is another reason NOT to allocate all 
your disk space Just Because It Is There. Keeping file systems mounted RO or not mounted helps keep them from 
needing to be fsck(8)ed after tripping over the power cord. 
Don't forget that if you have multiple disks on the system, they could all end up being fsck(8)ed after a crash at 
the same time, so they could require more RAM than a single disk. 
By the time one gets to somewhat larger than 1TB file system with default fragment and block sizes, fsck will 
require 1GB RAM to run, which is the application limit under OpenBSD. Larger fragments and/or blocks will 
reduce the number of inodes, and allow for larger file systems. 
FFS vs. FFS2
Using FFS, OpenBSD supports an individual file system of up to 2
31
-1, or 2,147,483,647 blocks, and as each 
block is 512 bytes, that's a tiny amount less than 1T. FFS2 is capable of much larger file systems, though other 
limits will be reached long before the file system limits will be reached. 
The boot/installation kernels only support FFS, not FFS2, so key system partitions (
/, /usr, /var, /tmp
) 
should not be FFS2, or severe maintenance problems can arise (there should be no reason for those partitions to 
be that large, anyway). For this reason, very large partitions should only be used for "non-system" partitions, for 
example, 
/home, /var/www/, /bigarray
, etc. 
Note that not all controllers and drivers support large disks. For example, 
ami(4)
 has a limit of 2TB per logical 
http://www.openbsd.org/faq/faq14.html (17 of 34)9/4/2011 10:02:25 AM

---

14 - Disk Setup
volume. Always be aware of what was available when a controler or interface was manufactured, and don't just 
rely on "the connectors fit".