Re: just how dangerous is this??

[Derek Vadala <derek@cynicism.com>]

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On Tue, 28 May 2002, [iso-8859-1] Jakob Østergaard wrote:

> Either it reconstructs it "right" or it does it "wrong".  It seems in
> his case it did it right.  Lucky him.
> 
> Hoever, the persistent superblocks will overwrite the last few KB of his
> *filesystem* on each partition.   So things *may* seem to work, but the
> system will fail horribly later.  After an fsck the RAID suprblocks will
> be damaged.  After another mkraid the filesystem will be damaged again.

You should be able to pre-plan by creating initial ext2 file systems that
are smaller than the partition size. You can do this by specifying the
number of blocks in the file system when you run mke2fs. 

mke2fs [options] device blocks

You should be able to calculate the eventual location of the md superblock
and select an appropriate block size for each partition to insure that the
superlbock and the filesystem do not overlap. 

Even if you've had sucess thus far, it's likely that problems will arise
as the filesystem fills up-- when the last blocks are allocated, and they
overlap the md superblock.

This probably isn't worth the effort unless you are doing multiple
installs with the same partition layout and hardware, but it seems that it
might be the case here.

I think the formula should be something like:

blocks - (blocks % 64) - 64 = md offset

That's for 1k blocks... ( like from fdisk -l)

I'm a bit lazy to double check, but I think that formula works.

Then when creating a filesystem divide by block size and create a file
system that's the right size... 

---
Derek Vadala, derek@cynicism.com, http://www.cynicism.com/~derek

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