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Date: Tue, 15 Jan 2002 03:01:54 +0000
To: Alex.Wilkinson@dsto.defence.gov.au
Cc: hackers@freebsd.org, freebsd-questions@freebsd.org
From: Anthony Naggs <tony@ubik.demon.co.uk>
Subject: Re: Super Block
References: <20020115120052.B8750-100000@squirm.dsto.defence.gov.au>
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In article <20020115120052.B8750-100000@squirm.dsto.defence.gov.au>,
Wilkinson,Alex <Alex.Wilkinson@dsto.defence.gov.au> writes
>Howdy Crew,
>
>I am wanting to find out the significance of the Super Block, whether FreeBSD, 
>Linux, Solaris...
>whatever.
>
>I know:
>
>*  The Super Block contains critical data for the device's filesystem [but what 
>??].
>*  It is located on sectors 16 through 31 at the beggining of the device.

I think these vary with the file system.

>*  FreeBSD keeps an alternate SuperBlock at the begging of every cylinder group.
>*  The first alternate Super block on FBSD is at block 32.
>
>This is all the info I could scrounge up.
>
>What does the SuperBlock actually do ?
>Why is the SuperBlock so critical ?

When the file system driver starts it reads the SuperBlock to confirm
that the driver can recognise the disk.  E.g. checking for "magic
numbers".

The SuperBlock contains information on the disk area allocated to the
partition, free/used blocks, where to find the root directory, block
size, inode size, etc...  It also indicates whether the disk has been
modified, which can force fsck to run, this is cleared when the volume
is unmounted.


It is hard to say much more without knowing more about  your interest in
it.


Cheers,
      Tony

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