Does FreeBSD start slices at head boundaries?

Rick Miller vmiller at
Fri Jul 6 21:13:05 UTC 2012

I went through this exercise to determine if there were boundary
issues installing FreeBSD on disks.  I concluded that FreeBSD was
indeed installing at head boundaries.  A colleague then pointed me to
which calls into question whether sysinstall and fdisk really are
installing FreeBSD's slice at the 64th cylinder.  Should I be
concerned with this?

This came about due to a scenario where Linux would start its
filesystem at sector 63, right before the head boundary.  On I/O
intensive applications, it was common for reads/write to cross the
head boundary resulting in unnecessary disk thrashing and long I/O
wait times.  The issue was corrected in Linux by changing the start
cylinder to 2048.  Some theorized that FreeBSD was vulnerable to this


On Fri, Jul 6, 2012 at 11:09 AM, Rick Miller <vmiller at> wrote:
> Hi All,
> Installing FreeBSD 8.x I select "A" at the fdisk partition editor to
> use the entire disk.  It creates an unused slice with offset 0 and 63
> sectors in size.  Then partition 1 starts at sector 63 and utilizes
> the remaining disk space.  Does sysinstall's diskPartitonEditor macro
> automatically start partitions at head boundaries?  The reason I ask
> is because I am most familiar with sector 64 being the start of a head
> boundary as opposed to 63.  Is my understanding incorrect?
> --
> Take care
> Rick Miller

Take care
Rick Miller

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