On 09/18/2014 12:57 PM, Rick Stevens wrote:
On 09/18/2014 11:45 AM, jd1008 issued this missive:
I have been searching for ext2/3 design docs to see where the on-disk
superblock is located relative to starting sector of the partition,
and how
many bytes it occucpies.
I was also looking for the formula that is used to compute the location
of all of the backup superblocks.
Thnaks for any pointers to online docs.
Try "man fsck.ext3" As part of the explanation of the "-b" option:
"For filesystems with 1k blocksizes, a backup superblock can be found at
block 8193; for filesystems with 2k blocksizes, at block 16384; and for
4k blocksizes, at block 32768."
Note that there are multiple alternate superblocks. For a 2K blocksize,
alternate superblocks should be available at 16384, 32768, 65536, etc.
(powers of 2). The best bet is to look at the output of mke2fs when the
filesystem was created (you do save that stuff, don't you?). If not and
you know the block size that was used, you can:
mke2fs -n -b <blocksize>
That will cause mke2fs to simulate making a filesystem based on that
blocksize. The "-n" means "don't actually make a filesystem, just
display what WOULD happen", so you'd get a list of where the alternate
superblocks would be located.
Thanks for the pointers Rick.
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