Installer: no option to set geometry/corrupts partition table

Matthew Dillon dillon at
Sun Mar 4 11:41:39 PST 2007

:Hi Matt,
:There needs to be an option such as FreeBSD (and NetBSD and iirc 
:OpenBSD) are providing, to set c/h/s for use by the DFly installer. I 
:don't understand the comment that nobody is using it- all the *BSD show 
:their view of it and if it doesn't match the partition table (which is 
:laid out to this day in terms of c/h/s whether it's actual or virtual) 
:it causes problems.
:For example, my machines are set up with 255 heads 63 sectors and how 
:ever many cylinders the drive maps to.  I fdisk the drive and I create 
:my partitions aligned on cylinder boundaries. When DFly installs it uses 
:16 heads and some other factor for sectors. He changed the units on the 
:partition table and this means that the partitions I had created before 
:and after (locations, not times) the DFly target slice are no longer on 
:integral boundaries, and that writing on the DFly filesystems can leak 
:into my other partitions and corrupt data.
:There should be a simple way to set the c/h/s in the installer. 
:Otherwise it does not play nice in a multiboot scenario.

    Well, my understanding is that tha partition table (the DOS parition
    table) stores linear block values in addition to CHS values.  If you
    tell the BIOS that the disk is in LBS or LARGE mode, the BIOS should 
    use the linear block values.  CHS values are undependable no matter what
    you do.  There is simply no proper way to set them that works with
    all BIOSes.

					Matthew Dillon 
					<dillon at>

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