[ILUG-BOM] Restoring the filesystem from alternate superblocls

Sameer D. Sahasrabuddhe sam_buddhe@[EMAIL-PROTECTED]
Sat Dec 15 13:39:03 IST 2001


erle wrote:

>take this for e.g. ( from: LINUX (UNIX) Unleashed, System Administrator's
>Edition )
>these are given as an e.g.
>
>8193,16385,24577,32769,40961,49153,57345,65537,73729.......

That is because the book assumes each block group consists of 8192 (2^13) 
blocks ...

>-- None of these worked... what worked for me was 32768.
>Just installed Debian .. the second backup block for me starts @ 32768. Next
>is 65536. .. nothing in between.

That is because Debian apparently uses 65536 blocks (2^16) in each group. I 
now remember I had used 65536 as the place for the alternate superblock on 
my RHL machine back then ... it seems convenient to use, since it will work 
for both, Debian machines as well as RHL. Point to note is that these are 
all just default numbers - they can be different in case of custom 
formatted filesystems. But using higher multiples will almost always work, 
since they are likely to encompass even the filesystems with smaller block 
group sizes ...

> >Vinay Pai wrote:
> >superblocks should be documented somewhere. If not, it shouldn't be too
> >hard to look at the source of mke2fs to determine it.
>
>that  _might_  be possible... havent found any available program to
>determine the same. Would be interesting. Maybe we should have a go @
>writing such a program.... it could be useful.

Superblock positions solely depend on the size of the block group used, 
which IIRC, can be specified as a commandline argument to mke2fs ... So its 
basically just a matter of remembering the size that is reported at the 
time of formatting, which, as Erle remarked, isn't something that we take 
too seriously :-)

SameerDS.

--
MTech Student,
Reconfigurable Computing Lab,
KReSIT, IIT-Bombay


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