Re: Reserved first sector for UFS

[Vladimir 'φ-coder/phcoder' Serbinenko]

On 01/07/2011 03:18 PM, Grégoire Sutre wrote:
> On 01/07/2011 13:20, Vladimir 'φ-coder/phcoder' Serbinenko wrote:
>
>>> Do you know of any OS that would put the superblock in sector 0?
>>> I  googled a bit, but I couldn't find examples where UFS would
>>> not start with a boot sector (afaics, it usually starts with a
>>> bootblock area of at least 8KiB -- with OS-specific data in it,
>>> e.g. a disklabel).
>>>
>> According to *BSD http://fxr.watson.org/fxr/source/ufs/ffs/fs.h:
>>
>> */"* Depending on the architecture and the media, the superblock may/*
>> */* reside in any one of four places. For tiny media where every
>> block /*
>> */* counts, it is placed at the very front of the partition.
>> Historically,/*
>> */* UFS1 placed it 8K from the front to leave room for the disk label
>> and/*
>> */* a small bootstrap. For UFS2 it got moved to 64K from the front to
>> leave/*
>> */* room for the disk label and a bigger bootstrap, and for really
>> piggy/*
>> */* systems we check at 256K from the front if the first three fail/*"
>
> Interesting.  So, when searching for a UFS superblock, sector 0 is a
> candidate (after sectors 128 and 16).  It seems to me that this use of
> sector 0 for tiny media does not apply to (recent) NetBSD newfs(8),
> though.
>
> If we want to improve the situation, we could let the filesystem driver
> dynamically decide whether the first sector is reserved or not.  But
> I'm not sure that it's worth it (the first sector of a partition
> containing a filesystem is only used for blocklist embedding, right?).
>
It's used for putting boot.img into partition. But you're right, it's
used mainly for blocklists installs.
> Grégoire
>
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-- 
Regards
Vladimir 'φ-coder/phcoder' Serbinenko

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