Curious about MBR

Benanzo · 07-20-2007, 02:21 AM

I have a question about the data that is stored in the MBR on my disk and individual partitions. I am not having any specific problems, I am just curious about some apparent discrepencies.

I have my disk split into four partitions:

Code:

Disk /dev/sda: 160GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags
 1      32.3kB  10.2GB  10.2GB  primary  ext3              
 2      10.2GB  12.3GB  2097MB  primary  linux-swap        
 3      12.3GB  150GB   137GB   primary  ext3              
 4      150GB   160GB   10.5GB  primary  ext3         boot

Feisty / (root) installed on /dev/sda1, swap is /dev/sda2, /home is /dev/sda3 and I have Gutsy installed on /dev/sda4. I have been playing around and investigating the MBR data on my disk recently and found something that perplexes me just a little.

When I use dd to copy out the MBR for the disk (/dev/sda) and then for each partition (/dev/sda1 sda2 etc) and then get info about the file using file I get this:

/dev/sda

Code:

mbr_sda: x86 boot sector; partition 1: ID=0x83, starthead 1, startsector 63, 19920537 sectors; partition 2: ID=0x82, starthead 254, startsector 19920600, 4096575 sectors; partition 3: ID=0x83, starthead 254, startsector 24017175, 268076655 sectors; partition 4: ID=0x83, active, starthead 254, startsector 292093830, 20482875 sectors, code offset 0x48

/dev/sda1 (Feisty / (root)

Code:

mbr_sda1: x86 boot sector, code offset 0x48

/dev/sda2 (swap)

Code:

mbr_sda2: data

/dev/sda3 (Feisty /home)

Code:

mbr_sda3: data

/dev/sda4 (Gutsy)

Code:

mbr_sda4: data

My question is regarding the lack of anything that indicates "active" or "bootable" on the Gutsy partition. parted clearly shows that the boot flag is set, but the MBR just says data. However, the boot flag is not set on /dev/sda1 (Feisty) but the MBR indicates that it contains a boot sector.

Am I misunderstanding the way this works? Like I said, I am experiencing no trouble booting into either Feisty or Gutsy with GRUB. I am just curious so I can understand what is going on.

Thanks!

bigrigdriver · 07-20-2007, 07:48 AM

Read these articles for an explanation of MBR.

http://www.cpqlinux.com/mbr.html
http://wiki.linuxquestions.org/wiki/Partition_table

syg00 · 07-20-2007, 08:19 AM

As it is generally referred to, the partition table for a disk only exists in the first sector of that disk. This contains all the relevant flags for (primary) partitions. Partitions themselves have equivalent records, but are boot sector records, and generally don't contain much of interest.
Logical partitions do use the partition table structure, but only for defining themselves, and allow chaining to the next logical partition. The whole extended/logical design was a bit of a hack added later to allow more than 4 partitions on a disk.

Stick to the partition table in the (real) MBR.

michaelk · 07-20-2007, 08:26 AM

linux does not use the boot flag.
Each partition contains a boot sector.

pixellany · 07-20-2007, 08:32 AM

The MBR is the first sector of the drive and is not part of any partition. Assuming just one drive, there MUST be boot code here to boot from the hard drive.

Each partition has what is usually called the "boot sector" which is the first sector of the partition. This can also contain boot code---e.g. when using chain-loading to boot multiple OSes.

In either case, the sector (512 bytes), breaks down as follows:
446 bytes boot code
64 bytes partition table
2 bytes "signature"

Primary and Extended partitions are listed in the MBR. Logicals are listed in the appropriate boot sector.

This is old, but is still useful:
http://www.ata-atapi.com/hiw.htm

Also, my site has articles on booting and partitions

Benanzo · 07-20-2007, 03:13 PM

Wow, a lot a great info.

Thanks!