300Gb hard drive formatting to 249Gb - boo.

[Malcolm Kay]

On Mon, 6 Dec 2004 08:33 pm, Alex Teslik wrote:
> Hello,
>
>    I have recently installed a new 300Gb Maxtor hard disk:
>
> [root]/home/alex# dmesg -a | grep ^ad1
> ad1: 286188MB <Maxtor 6B300R0> [581463/16/63] at ata0-slave UDMA100
>
> This disk has a single partition on it and is formatted in compatibility
> mode:
>
> [root]/home/alex# disklabel ad1
> # /dev/ad1c:
> type: ESDI
> disk: ad1s1
> label:
> flags:
> bytes/sector: 512
> sectors/track: 63
> tracks/cylinder: 255
> sectors/cylinder: 16065
> cylinders: 36482
> sectors/unit: 586099332
> rpm: 3600
> interleave: 1
> trackskew: 0
> cylinderskew: 0
> headswitch: 0           # milliseconds
> track-to-track seek: 0  # milliseconds
> drivedata: 0
>
> 8 partitions:
> #        size   offset    fstype   [fsize bsize bps/cpg]
>   c: 586099332        0    unused        0     0        # (Cyl.    0 - 36482*)
>   e: 586099332        0    4.2BSD     1024  8192    22  # (Cyl.    0 - 36482*)
>

You'll notice that this (partition e) also calculates out to 279Gb (base 1024)

> now, I understand that a Gigabyte to the Maxtor corporation is
> 300,000,000,000 bytes. So, I would expect this disk to format to 279Gb
> based on the following math:
>
> [alex]/home/alex# bc
> bc 1.06
> Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc.
> This is free software with ABSOLUTELY NO WARRANTY.
> For details type `warranty'.
>
> (((300000000000 / 1024) /1024) /1024)
> 279
>
> However, when I mount the drive it is formatted to 271Gb, 8Gb less than
> what I expected. This wouldn't be so bad except that the available space on
> the drive is only 249Gb. :(
> Is this right? I feel like I am missing something. Can someone help me
> understand this better? I would love to regain the 30Gb I feel I am missing
> if possible.
>
> [root]/home/alex# mount /dev/ad1s1e /1
>
> [root]/home/alex# df -h
> Filesystem    Size   Used  Avail Capacity  Mounted on
> /dev/da0s1a    97M    62M    27M    70%    /
> /dev/da0s1e   7.7G   6.2G   938M    87%    /usr
> /dev/ad0s1e    72G    66G   817M    99%    /home
> procfs        4.0K   4.0K     0B   100%    /proc
> /dev/ad1s1e   271G   1.0K   249G     0%    /1
>
This (271Gb) is the formated capacity that left is after the file system 
overheads have been allocated. The amount used will depend in part on the number 
of inodes made available during creation of the fs. 

The 249Gb excludes a minumum free space required for efficient (speedy)
disk operation. Part of the reserve may sometimes actually be used by privileged
processes with the Capacity figure exceeding 100%. The reserve as a percentage 
of the total is setable -- see tunefs(8).

Malcolm