New HOWTO: Linux Partition HOWTO - page 7

Table of Contents

6. Recovering a Deleted Partition Table

 1. Make a partition that is at least as big as your first partition was. You
    can make it larger than the original partition by any amount. If you
    underestimate, there will be much wailing and gnashing of teeth.
    Command (m for help): n                                                    
    Command action                                                             
       e   extended                                                            
       p   primary partition (1-4)                                             
    p                                                                          
    Partition number (1-4): 1                                                  
    First cylinder (1-23361, default 1):                               
    Using default value 1                                                      
    Last cylinder or +size or +sizeM or +sizeK (1-22800, default 22800): 13032 
                                                                               
    Command (m for help): w                                                    
   
 2. Run dumpe2fs on the first partition and grep out the block count.
   
    Example:
               % dumpe2fs /dev/sda1 | grep "Block count:"            
               Block count:              41270953                    
                                                                     
    If you are uncertain about this value, repeat Step 1 with a bigger
    partition size. If the block count changes, then you underestimated the
    size of the original partition. Repeat Step 1 until you get a stable
    block count.
   
 3. Remove the partition you just created
             Command (m for help): d                                 
             Partition number (1-4): 1                               
                                                                     
   
 4. Make a new partition with the exact size you got from the block count.
    Since you cannot enter block size in fdisk, you need to figure out how
    many cylinders to request. Here is the formula:
   
      (number of needed cylinders) = (number of blocks) / (block size)                                                 
                                                                                                                       
      (block size) = (unit size) / 1024                                                                                
                                                                                                                       
      (unit size) = (number of cylinders) * (number of heads) * (number of sectors/cylinder) * (number of bytes/sector)
    In theory! In practice, it's rather more complicated. fdisk tries to end
    its allocation for a partition on a cylinder boundary, so it can be hard
    to figure out the relationship of cylinders to blocks.
   
    Here is an example of the problem. Below, I have formatted a drive with
    partitions with 1, 2, 4, and 8 cylinders.
    disk /dev/sda: 16 heads, 63 sectors, 23361 cylinders             
    Units = cylinders of 1008 * 512 bytes                            
                                                                     
       Device Boot    Start       End    Blocks   Id  System         
    /dev/sda1             1         2       976+  83  Linux          
    /dev/sda2             3         5      1512   83  Linux          
    /dev/sda3             6        10      2520   83  Linux          
    /dev/sda4            11        19      4536   83  Linux          
    If I divide each partition by the number of cylinders, I ought to get the
    block size (16 heads * 63 sectors * 512 bytes/sector divided by 1024 =
    504), right? Not true!
            allocated  #of  block                                    
             blocks    cyl  size                                     
                                                                     
               976 /    1 = 976                                      
              1512 /    2 = 756                                      
              2520 /    4 = 630                                      
              4536 /    8 = 567                                      
              8568 /   16 = 535                                      
             16632 /   32 = 519                                      
             32760 /   64 = 512                                      
             64984 /  128 = 507                                      
            129528 /  256 = 505                                      
            258552 /  512 = 504                                      
            516600 / 1024 = 504                                      
           1032664 / 2048 = 504                                      
    Notice that as the number of cylinders grows, the closer to the real
    block size the calculated value for the allocated blocks becomes.
   
    You will have to make guestimates and converge on the true number of
    cylinders to use. You will ultimately get an exact match because the
    block count from dumpe2fs came from a well-formed partition.
   
 5. Run e2fsck on it to verify that you can read the new partition.
   
 6. Repeat Steps 1-5 on remaining partitions.
   

Remount your partitions. Amazingly, all of your data will be there.

Credit goes to:

  * Mike Vevea, jedi sys admin and MGH's finest, for giving me these tips.
   

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