[GUIDE] SD card partitioning for rooted phones

Requirements

• rooted phone
• busybox installed
• parted (optional)
• backup your SD card (optional)
• calculator

Click to expand...

Click to collapse

Background

Before we begin partitioning, we need to elaborate some key points:

1. block storage units are divided into logical blocks known as sectors
2. sector has a size of 512 bytes
3. NAND flash chips are divided into blocks known as erase blocks
4. our SD cards consist of those NAND flash chips and controller
5. erase block on our SD cards has a size of 128 kB, that's 256 sectors
6. CHS (cylinder, head, sector) alignment has an insignificant importance here
7. 1st sector is sector 0 (not 1) and is used as MBR (master boot record)
8. 1st partition begins at cylinder boundary to maintain MS-DOS compatibility
9. raw access to block storage units is done via special block device files under /dev/block directory
10. our SD card is represented by block device file /dev/block/mmcblk0

Click to expand...

Click to collapse

Instructions

Here I will provide you with two methods of partitioning. For 1st method you will be using fdisk utility which is part of busybox and for 2nd a standalone utility called parted will be used. Both methods can be used in normal mode via adb shell or some terminal app. I will explain both methods using adb shell as it is more convenient and handy than typing commands via touch keyboard on your phone.

1st thing to do before you begin is to unmount your SD card via "Settings->SD & phone storage" and then you issue "adb shell" command ony your PC. 2nd thing you will do is erasing of your SD card (actually you will erase just first few erase blocks of your SD card) using dd utility:

Code:

dd if=/dev/zero of=/dev/block/mmcblk0 bs=131072 count=16

...that will overwrite 1st 2 MB of your SD card with null characters. Next you may begin with partitioning.

fdisk
As I already stated, fdisk is a (interactive) utility that is part of busybox so I will assume it is available under /system/xbin directory. Now you can run fdisk with device file of your SD card as parameter/argument:

Code:

fdisk /dev/block/mmcblk0

...this will bring you some notes on your screen you should not worry about and a command prompt:

Code:

Command (m for help):

...which you can leave at any time by pressing CTRL+C. Next you will change unit display type to sectors:

Code:

Command (m for help): [B]u[/B]
Changing display/entry units to sectors

...and print your SD's current info (this is info of my SD card actually, yours may vary):

Code:

Command (m for help): [B]p[/B]

Disk /dev/block/mmcblk0: 8018 MB, 8018460672 bytes
4 heads, 16 sectors/track, 244704 cylinders, total 15661056 sectors
Units = sectors of 1 * 512 = 512 bytes

              Device Boot      Start         End      Blocks  Id System

...and you write down the number of sectors. In my case it is 15661056 sectors of 512 bytes which is exactly 7647 MB if we divide them by 2048. For example you would take 7000 MB for fat32 1st partition and 647 MB for ext 2nd partition. and it is handy that way coz megabytes are divisible by our SD card's erase block size which is 128 kB as stated before. Calculation would give you start sector for 2nd partition and this would be 14336000 (7000*2048).

Now you need to create 2 primary partitions:

Code:

Command (m for help): [B]n[/B]
Command action
   e   extended
   p   primary partition (1-4)
[B]p[/B]
Partition number (1-4): [B]1[/B]

...now there's a catch. You will be offeread a start of 1st partition at 1st to 2nd cylinder boundary which is sector 16 in my case and you push it to SD card's erase block boundary (256):

Code:

First sector (16-15661055, default 16): [B]256[/B]
Last sector or +size or +sizeM or +sizeK (256-15661055, default 15661055): [B]14335999[/B]

...and continue to the next partition which should also be primary:

Code:

Command (m for help): [B]n[/B]
Command action
   e   extended
   p   primary partition (1-4)
[B]p[/B]
Partition number (1-4): [B]2[/B]
First sector (16-15661055, default 16): [B]14336000[/B]
Last sector or +size or +sizeM or +sizeK (14336000-15661055, default 15661055): [B]15661055[/B]

...now print what you have just done:

Code:

Command (m for help): [B]p[/B]

Disk /dev/block/mmcblk0: 8018 MB, 8018460672 bytes
4 heads, 16 sectors/track, 244704 cylinders, total 15661056 sectors
Units = sectors of 1 * 512 = 512 bytes

              Device Boot      Start         End      Blocks  Id System
/dev/block/mmcblk0p1             256    14335999     7167872  83 Linux
/dev/block/mmcblk0p2        14336000    15661055      662528  83 Linux

...it looks OK but you need to change 1st partition's hex id which needs to be fat32 (c):

Code:

Command (m for help): [B]t[/B]
Partition number (1-4): [B]1[/B]
Hex code (type L to list codes): [B]c[/B]
Changed system type of partition 1 to c (Win95 FAT32 (LBA))

...now you're am set, print again your configuration and write changes to SD card:

Code:

Command (m for help): [B]p[/B]

Disk /dev/block/mmcblk0: 8018 MB, 8018460672 bytes
4 heads, 16 sectors/track, 244704 cylinders, total 15661056 sectors
Units = sectors of 1 * 512 = 512 bytes

              Device Boot      Start         End      Blocks  Id System
/dev/block/mmcblk0p1             256    14335999     7167872   c Win95 FAT32 (LBA)
/dev/block/mmcblk0p2        14336000    15661055      662528  83 Linux

Command (m for help): [B]w[/B]
The partition table has been altered!

There's a possibility you would need to shutdown and power on again your phone at this point. Do not reboot via adb or some 3rd party app!

Click to expand...

Click to collapse

parted
Parted is one of interactive partitioning utilities that can also use external formatting utilities. It can be found in some recovery images but can be copied to your internal phone storage and run from there in normal mode too. To run it you have to use your SD card's device file as a parameter/argument:

Code:

parted /dev/block/mmcblk0

...and you will be presented with an interactive shell:

Code:

GNU Parted 1.8.8.1.179-aef3
Using /dev/block/mmcblk0
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted)

I probably shouldn't mention that there's an interactive help available and that it is invoked by issuing "help" into shell's command prompt. Next thing to do is making a MS-DOS disklabel:

Code:

(parted) [B]mklabel msdos[/B]

...and switch to display sector as a unit:

Code:

(parted) [B]unit s[/B]

Now you can print some useful info:

Code:

(parted) [B]print all[/B]
Model: SD USD (sd/mmc)
Disk /dev/block/mmcblk0: 15661056s
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start      End        Size       Type     File system  Flags

Mind and write down the size in sectors (15661056 in my case).If you divide number of sectors by 2048, you get how big in MB is actually your SD card (7647 in my case).You should mind that erase block of your SD card is 128 kB and all of your partitions should start at the beginnings of those erase blocks. It is safe to say that 1st partition should begin at sector 256 and 2nd at any MB boundary. Let say you want 512 MB big ext partition and the rest for fat32 one. Mind tho that 1st partition is to be fat32! So we say 7135 MB for fat32 1st partition and 512 MB for ext 2nd partition. Now you calculate the start sector of 2nd partition... number of MB for 1st partition multiplied by 2048 should give you the number (14612480). And you are set for partitioning:

Code:

(parted) [B]mkpart primary fat32 256 14612479[/B]
(parted) [B]mkpart primary ext2 14612480 15661055[/B]

...and print result:

Code:

(parted) [B]print all[/B]
Model: SD USD (sd/mmc)
Disk /dev/block/mmcblk0: 15661056s
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start      End        Size       Type     File system  Flags
 1      256s       14612479s  14612224s  primary  fat32        lba
 2      14612480s  15661055s  1048576s   primary  ext2

...and quit:

Code:

(parted) [B]quit[/B]

Click to expand...

Click to collapse

At this point you have partitioned your SD card but not yet formatted it. Format fat32 partition with mkfs.vfat and ext partition with mkfs.ext2:

Code:

mkfs.vfat /dev/block/mmcblk0p1

...and:

Code:

mkfs.ext2 -m0 -b4096 /dev/block/mmcblk0p2

If there is a mke2fs utility on your phone system (standalone - not part of busybox), you may use it to format second partition as ext3:

Code:

mke2fs -j -m0 -b4096 /dev/block/mmcblk0p2

...or even as ext4 (if your mke2fs supports that):

Code:

mke2fs -j -m0 -b4096  -Oextents,uninit_bg,dir_index /dev/block/mmcblk0p2

Click to expand...

Click to collapse

tasar said:

Thanks, with the "u" option comes later in your manual

Click to expand...

Click to collapse

A last newbie question: i have now 2 part. and formated the FAT, but i dont know, how to get the "mke2fs" on the phone to format the Linux part.?
Sorry Blay0 but Linux is another Word for me...

Click to expand...

Click to collapse

playahate said:

Need some help.

Code:

Command (m for help): p
p

Disk /dev/block/mmcblk0: 7973 MB, 7973371904 bytes
4 heads, 16 sectors/track, 243328 cylinders
Units = cylinders of 64 * 512 = 32768 bytes

              Device Boot      Start         End      Blocks  Id System

What is my mistake?

Click to expand...

Click to collapse