Malcolm Kay malcolm.kay at
Sun Jan 15 23:20:53 PST 2006

On Mon, 16 Jan 2006 04:03 am, Jona Joachim wrote:
> Hi!
> I'm quite new to FreeBSD and I have some concerns about
> partitioning. I plan to install FreeBSD-6 as a desktop system
> and I bought a new Western Digital Caviar SE WD800JB hard
> drive to use it as a second disk for storing music, movies,
> photos and stuff like that on it. I also want to put the swap
> partition on it as this drive will be idle most of the time.
> So basically I want to create 2 partitions on this drive: one
> 2GB partition for swap and one big UFS partition for data.
> Fdisk reported that it couldn't clearly identify the disk
> architecture because it thinks that the architecture is
> different from what BIOS tells. Here is fdisk's output after
> having created a slice:

With large modern disks the "architectures" quoted are mostly 
ficticious bearing little are no relationship to the physical 
structure of the disk. With LBA addressing the precise 
ficticious architecture used is mostly of little significance.

> ******* Working on device /dev/ad1 *******
> parameters extracted from in-core disklabel are:
> cylinders=155061 heads=16 sectors/track=63 (1008 blks/cyl)
> Figures below won't work with BIOS for partitions not in cyl 1
> parameters to be used for BIOS calculations are:
> cylinders=155061 heads=16 sectors/track=63 (1008 blks/cyl)
> Media sector size is 512
> Warning: BIOS sector numbering starts with sector 1
> Information from DOS bootblock is:
> The data for partition 1 is:
> sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD)
>     start 0, size 156296322 (76316 Meg), flag 80 (active)
> 	beg: cyl 0/ head 0/ sector 1;
> 	end: cyl 512/ head 253/ sector 63
> The data for partition 2 is:
> The data for partition 3 is:
> The data for partition 4 is:
> After creating a slice I created the partitions inside it with
> disklabel. In every example I saw the swap partition was in
> the second place. However, I want it to be in the beginning of
> the disk as this should result in better performance. Are
> there any concerns about this layout? I basically used the
> standards mentioned by the manual pages of disklabel:
> 8 partitions:
> #         size   offset    fstype [fsize   bsize bps/cpg]
>   a:         2g          *      swap
>   b:           *          *  4.2BSD  4096 16384          64
>   c:           *          *   unused

Partition 'a' for root and 'b' for swap is a strongly established 
convention and departure could lead to all sorts of hassles.

If you must have swap earlier than the root partition then retain 
the labels but change the positions -- that is set the offset of 
'a' so that it falls after 'b' and set the 'b' partition at or 
near the beginning of the slice. 

To me it it is not entirely clear whether reservations for things 
like disklabels are not automatically taken care of when 'a' is 
the first partition in the slice, so I suggest if you place the 
'b' partition first you do this with an offset of 16 sectors.

But make it a rule to retain the traditional relationship between 
partition names and usage; just move the partition allocation.

Malcolm Kay
> Here is an output of what the layout now actually looks like:
> # /dev/ad1s1:
> 8 partitions:
> #        size                 offset        fstype   [fsize
> bsize bps/cpg] a:  4194304                    0          swap
>   b: 152102018     4194304      4.2BSD     4096 16384 20732
>   c: 156296322                 0       unused           0     
>    0
> The number of cylinders in a cylinder group bps/cpg was
> automatically changed to 20732 which is quite different from
> 64. Why is that so? I haven't had any problems with the disk
> until now but I don't want to expose my data to danger by
> using a broken disk layout. Thanks for your help!

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