Aug 27, 2012

GPT, beyond the MBR (II)

Let's follow up on the second part of the article about GPT, beyond the MBR.

Now, a new GPT partition table is going to be created by means of parted, and afterwards, it will be displayed.

root@ubuntu-server:~# parted /dev/sdb mklabel gpt

root@ubuntu-server:~# parted /dev/sdb print
Model: VMware, VMware Virtual S (scsi)
Disk /dev/sdb: 1074MB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start  End  Size  File system  Name  Flags

If you try to edit the partition table with fdisk, you will come across a message as follows.

root@ubuntu-server:~# fdisk /dev/sdb 

WARNING: GPT (GUID Partition Table) detected on '/dev/sdb'! The util fdisk doesn't support GPT. Use GNU Parted.
...

Let's create for example ten primary partitions of 10 MB each through a simple bash script.

root@ubuntu-server:~# j=1 ; for ((i=11; i<=101; i+=10)); do parted /dev/sdb mkpart primary $j $i; j=$i ; done

root@ubuntu-server:~# parted /dev/sdb print
Model: VMware, VMware Virtual S (scsi)
Disk /dev/sdb: 1074MB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start   End     Size    File system  Name     Flags
 1      1049kB  10.5MB  9437kB               primary
 2      10.5MB  21.0MB  10.5MB               primary
 3      21.0MB  31.5MB  10.5MB               primary
 4      31.5MB  40.9MB  9437kB               primary
 5      40.9MB  51.4MB  10.5MB               primary
 6      51.4MB  60.8MB  9437kB               primary
 7      60.8MB  71.3MB  10.5MB               primary
 8      71.3MB  80.7MB  9437kB               primary
 9      80.7MB  91.2MB  10.5MB               primary
10      91.2MB  101MB   9437kB               primary

If we take a look at the GPT table, we can distinguish the following parts.

root@ubuntu-server:~# dd if=/dev/sdb bs=512 count=4 | xxd -c 16
0000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
...
00001f0: 0000 0000 0000 0000 0000 0000 0000 55aa  ..............U.
0000200: 4546 4920 5041 5254 0000 0100 5c00 0000  EFI PART....\...
...
0000400: a2a0 d0eb e5b9 3344 87c0 68b6 b726 99c7  ......3D..h..&..
0000410: 4e6e ad36 2fec 8046 bc1f 4a42 82d2 8052  Nn.6/..F..JB...R
0000420: 0008 0000 0000 0000 ff4f 0000 0000 0000  .........O......
0000430: 0000 0000 0000 0000 7000 7200 6900 6d00  ........p.r.i.m.
0000440: 6100 7200 7900 0000 0000 0000 0000 0000  a.r.y...........
...

First up, it is the legacy MBR, which GPT holds for reasons of compatibility (the code 0x55AA points to the end of the MBR). The second part of the GPT (512 bytes) contains the header information for GUID partitioning. And the first partition entry appears in position 0x400.


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