Data Partition

From Entuura

For OpenWRT on Alix, we like the fact that the install image is one little squashfs, and that it does not care about the actual size of the CF card where it lives. We'd like to have a data partition live out at the END of the CF card, so that the OS image can be reapplied and will "re-find" the data partition.

Here are some notes on how to do this.

root@Entuura-Leeds:/proc# cat partitions 
major minor  #blocks  name
   3        0     990864 hda
   3        1       4504 hda1
   3        2      49360 hda2
  31        0      49344 mtdblock0
  31        1      39552 mtdblock1
   7        0     754416 loop0

This lets us easily see if the data partition exists yet, and also gives us the number of blocks of the CF card. This is a 1 gig card, so these blocks must be 1024 bytes.

dmesg says:

 hda: 1981728 sectors (1014 MB) w/1KiB Cache, CHS=1966/16/63

Sectors are 512 bytes, and sure enough, 1981728 (number of sectors reported by fdisk) is double 990864 (number of blocks reported by /proc/partition).

fdisk says this:

root@Entuura-Leeds:/proc# fdisk -l -u /dev/hda
Disk /dev/hda: 1014 MB, 1014644736 bytes
16 heads, 63 sectors/track, 1966 cylinders, total 1981728 sectors
Units = sectors of 1 * 512 = 512 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/hda1   *          63        9071        4504+  83  Linux
/dev/hda2            9135      107855       49360+  83  Linux

So, it all comes together. The length of hda2 is 107855 - 9135 = 98720 sectors. Divided by two, that's 49360 blocks, like like /proc/partitions says.

Finally, we need a way to figure out the highest sector number that fdisk will take. That comes from doing it all backwards. /proc/partition says that hda has 990864 blocks. Multiply it by two and you get 1981728. Sectors are zero-based, so subtract one. The higest sector number is 1981727, just like fdisk thinks:

root@Entuura-Leeds:/sys/module/block2mtd/parameters# fdisk -u /dev/hda   
Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p  
Partition number (1-4): 3
First sector (9072-1981727, default 9072):

Yay, so putting it all together we:

• get length of hda from /proc/partitions
• multiply by 2, minus 1 to get the end sector
• choose an appropriate buffer after hda2 for start sector
• put all of it into fdisk (but actually, you can just let fdisk calculate the end sector itself if you want)

Here's what goes into the UCI-managed fstab:

fstab.@mount[0]=mount
fstab.@mount[0].fstype=ext3
fstab.@mount[0].options=rw,sync
fstab.@mount[0].enabled=1
fstab.@mount[0].device=/dev/hda3
fstab.@mount[0].target=/data/fixed

The code that does this stuff is in init.d/et-startup.