Re: A tool for remapping bad sectors in CURRENT?

On Sun, 14 Mar 2010 17:18:45 +0100
Miroslav Lachman <000.fbsd_at_quip.cz> wrote:

> Gary Jennejohn wrote:
> > On Sun, 14 Mar 2010 10:55:19 +0100
> > Miroslav Lachman<000.fbsd_at_quip.cz>  wrote:
> >
> > [big snip]
> >> fsdb (inum: 3)>  blocks
> >> Blocks for inode 3:
> >> Direct blocks:
> >> 3001 (1 frag)
> >>
> >> fsdb (inum: 3)>  findblk 3001
> >> fsdb (inum: 3)>
> >>
> >>                  ^^^^^^^^ findblk did not returned inode 3!
> >>
> >
> > This is almost guaranteed to be a file system block and not
> > a disk block.
> 
> Do you mean the number 3001?
> I am sorry for my ignorance, but it is not clear to me from fsdb manpage 
> what "blocks" means FS block and what disk block.
> 
> And how can I use (calculate with) this numbers?
> 
> How can I get the right number to pass to findlbk command (in the 
> example above) to give me back the inode 3?
> 
> If FS block is 16384 bytes, then it means 16384/512 = 32 disk blocks per 
> FS block.
> 
> If 3001 is FS block, then it means 3001*32 = 96032 disk block number. Am 
> I right?
> 
> fsdb (inum: 3)> findblk 96032
> fsdb (inum: 3)>
> 
> Again - findblk did not returned inode 3.
> 
> So what is the exact formula to get the right findblk number and then 
> right inode number as result of findblk command?
> 
> I am still lost in terms (words) and numbers :(
> 

Well, it's pretty hairy.

Looking at findblk() it does this to go from disk block to file system
block (this is greatly simplified)

     file_system_blockno = disk_blockno >> fs_fsbtodb;

So conversely, you'd do disk_blockno = file_system_blockno << fs_fsbtodb.

You can get this information using "ffsinfo -l 0x001 -o some_file
/dev/ataXY" (using ahci) and grep'ing for fsbtodb in some_file. The
0x001 means to only dump the first super block.

I looked at a file system which has default 16kB file system blocks and
fsbtodb is 2 ==> *multiply file_system_block by 4 not 32*.  This is probably
because it's a multiple of a 4kB block, which is the smallest usable
file system block size AFAIK.

BTW looking at the code leads me to conclude that fsdb will not print out
anything if the disk block you're trying to find has bever been allocated
to an inode ==> unused disk block, safe to overwrite.  This assumes that
you calculated the disk block correctly.

---
Gary Jennejohn