Blew away my partition table

Jefferson Ogata poweredge at
Tue Jun 29 16:00:44 CDT 2010

On 2010-06-29 20:22, Eberhard Moenkeberg wrote:
> On Tue, 29 Jun 2010, Jefferson Ogata wrote:
>> You should be able to use dmsetup to create device nodes with offset
>> into /dev/sda if you want to do this. But you should be able to find
>> your filesystem headers with dd and xxd (or any hexdump program).
> A very good idea, to avoid the reboot.

Well, apparently using dmsetup doesn't work because the kernel refuses 
to set up new mappings directly on /dev/sda, possibly because there's an 
existing lock on the device due to the partition table being loaded.

>> Where to look:
>> - The first partition starts one track into the disk; typically that's
>> 63 512-byte sectors.
>> - The second, third, and fourth partitions are usually on cylinder
>> boundaries, with a cylinder typically being 63 * 255 512-byte sectors.
>> - If you had more than four partitions, then the last physical partition
>> has a partition table at the beginning. The first logical partition will
>> begin one track into that physical partition.
>> What to look for:
>> - For ext3 filesystems, a superblock begins 1024 bytes into the
>> partition. At offset 0x38 in the superblock you should find the magic
>> number 0x53ef (big-endian).
>> - For swap partitions, look at the first 4096 bytes. At the end of that
>> page you should find the string SWAPSPACE2.
>> - For LVM physical volumes you should see an LVM label 512 bytes from
>> the beginning of the partition.
> A nice collection. Thanks, I will keep it in case i get into partition 
> table trouble.

A little more info:

- For ext3 filesystems, the superblock begins with a series of uint32_ts 
in little-endian format. The second uint32_t is the number of filesystem 
blocks in the filesystem. The seventh uint32_t is the block size, 
expressed as the number of bits to shift 1024 left. (So 0 for 1024-byte 
blocks, 1 for 2048-byte blocks, 2 for 4096-byte blocks). From this you 
can calculate the offset to the next partition--multiply the number of 
blocks by the actual blocksize and round up to a cylinder boundary.

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