FDISK Discrepancies...FC12 vs. FC14

bmcclint · #1 28th November 2010, 12:12 AM

Not sure which forum topic this should probably be posted so I'll start here.

I was recently messing around with the partitions on my tri-boot system and ultimately messed everything up. After a few hours of tinkering I was able to recover but learned something interesting in the process.

Something has changed with FDISK, or some underlying component, from FC12 to FC14. I keep partition backups so when I mess things up, I restore using dd. Anyway I think I was restoring partitions at the wrong starting blocks as some partitions worked while others did not. I just happened to notice the start for sda1 on FC14 and said to myself, that's not right.

Anyway take a look as the 'fdisk -l /dev/sda' for FC12 and FC14. What gives? My backups were all made on an FC12 install and when I restored from an FC12 boot CD all is well.

Anyone know what the deal is? Should I recreate all my backups again using FC14? Not sure how the boot sector is affected with this FC14 sda1 start at 64.

Though?

HTML Code:

*** FC12 x64
Disk /dev/sda: 640.1 GB, 640135028736 bytes
255 heads, 63 sectors/track, 77825 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x00018726

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1        2612    20980858+   7  HPFS/NTFS
/dev/sda2            2613        5224    20980890   83  Linux
/dev/sda3   *        5225       10447    41953747+  af  Unknown
/dev/sda4           10448       77825   541213785    5  Extended
/dev/sda5           10448       10709     2104483+  82  Linux swap / Solaris
/dev/sda6           10710       77825   539109238+   7  HPFS/NTFS

*** FC14 x64
Disk /dev/sda: 640.1 GB, 640135028736 bytes
255 heads, 63 sectors/track, 77825 cylinders, total 1250263728 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x4d1d8a4d

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1              63    41961779    20980858+   7  HPFS/NTFS
/dev/sda2        41961780    83923559    20980890   83  Linux
/dev/sda3   *    83923560   167831054    41953747+  af  HFS / HFS+
/dev/sda4       167831055  1250258624   541213785    5  Extended
/dev/sda5       167831118   172040084     2104483+  82  Linux swap / Solaris
/dev/sda6       172040148  1250258624   539109238+   7  HPFS/NTFS

stoat · #2 28th November 2010, 12:30 AM

Quote:

Originally Posted by bmcclint

Anyone know what the deal is?

The Fedora 12 fdisk is using cylinders as the unit, and the Fedora 14 fdisk is using sectors. The fdisk default is now to use sectors instead of cylinders. See fdisk --help in each system and you'll probably see what I mean. To make the new system's fdisk display with cylinders, try this...

Code:

su
fdisk -l -u=cylinders

Anyway, your first partition starts at cylinder 1 which starts at sector 63. Very normal. Nothing is wrong. It's apparently something new about fdisk to get used to. I have to admit that I hadn't really noticed until you mentioned it. I often would make the old version display sectors with this...

Code:

su
fdisk -lu

So I'm already quite used to seeing 63 or 2048 as the beginning of the first partition in an fdisk output. I didn't really notice that it is now the fdisk default.

Quote:

Originally Posted by bmcclint

My backups were all made on an FC12 install and when I restored from an FC12 boot CD all is well.

Well, when you made your backups with dd, I'm sure you didn't concern yourself with the starting and ending sectors of partitions that you backed up. You probably designated the partition to be backed up as /dev/sdaX. So dd still did the right thing backing up and restoring, and fdisk just reported the partitions in different units in the two systems. Nothing to fix.

P.S.: I wonder if we now also will stop seeing fdisk report partitions not ending on cylinder boundaries because that standard went away quite a while ago. Only fdisk still cared about it. In fact, that may even by the reason for the change in the default unit. Lately, it has been common in an fdisk output to see a partition end in one cylinder and the next partition start in the very same cylinder. Displaying that situation in sectors instead will make more sense when partitions don't start and end at cylinder boundaries.

P.P.S.: In a legacy hard drive, the first sector is the master boot record (sector 0). The first partition starts in sector 63. Those 62 sectors between the master boot record and the first partition are known as the DOS Compatibility Region. It's normally not used for anything and was intended to satisfy the DOS requirement for partitions to begin on cylinder boundaries. Some applications will stick things in there. For example, it's the place where legacy GRUB writes stage1.5. GRUB for DOS will put code there, too. There is one other that I used to know about, but I can't remember it now.

bmcclint · #3 29th November 2010, 03:38 AM

Thanks for the response...seems to answer a few and I did change unit type and it looks the same.

Still confused when I restored using dd on FC14 nothing booted but when using FC12 everything came back to life. Maybe I just missed something.

Yes I just let DD backup the partition...
dd if=/dev/sda of=sda1.raw bs=64k
dd if=sda1.raw of=/dev/sda1 bs=64k