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Repartitioning the flash

Revision as of 11:03, 28 February 2010

Repartitioning the flash

There are several reasons why users might want to repartition their eMMC flash:

1. many Maemo power users have a Linux background. They tend to prefer POSIX filesystems like ext3 over the ancient VFAT filesystem. However, VFAT can be accessed on most non-Linux computers and would be useful to keep a small VFAT partition for data exchange with USB mass storage. With a large ext3 partition you could install much more applications, store symlinks and permissions (e.g., for backup of your desktop PC), or create Linux chroot enviroments (e.g., for installing non-optified software or Debian).
2. booting alternative operating systems (e.g., Mer,Moebian, Fedora) requires independent partitions
3. loopback files for chroots (see Easy Debian) can be stored in VFAT MyDocs, but access is much slower than on a native partition. loopback files in MyDocs are not available during USB mass storage mode.

By default the eMMC on the N900 (/dev/mmcblk0) is partitioned as follows:

1. 27GB VFAT MyDocs partition
2. 2GB /home partition
3. 768MB swap partition

This page describes several possible solutions:

1. Swap the 2GB /home and 27GB MyDocs partition to get a 2GB vfat MyDocs and a 27GB ext3 /home. No partition size need to be changed and only a single reboot is necessary. This is the least invasive solution and can be easily reverted. It leaves a smaller MyDocs partition for VFAT USB mass storage mode.
2. shrink MyDocs to add additional partitions (e.g., for other OS).
3. setup an arbitrary /home and MyDocs division of the space. this is a little bit more complicated.
4. get rid of the MyDocs partition, setup a single 31GB ext3 partition, and replace MyDocs with loopback file(s) that could also be USB exported and have arbitrary size and filesystem. This is useful if you rarely need USB mass storage mode or if you want to export different filesystem images depending on the computer. However, the loopback files may be significantly slower.

What you should know before partitioning

1. BACKUP YOUR DATA: a mistake during partitioning could brick your device, lead to complete data loss and could require reflashing. Use the Backup application to backup all settings etc. Backup this and the contents of MyDocs to your computer. Remove your optional SD card to prevent data loss. Following this tutorial should minimize your risk of data loss but repartitioning is at your own risk.
2. make sure your battery is not nearly empty (rebooting costs a lot of power) or, even better, plug in your charger.

some remarks

1. partitions on flash should be aligned to flash block sizes (AFAIK this is 32K, but some sources claims 64K or 128K). To be on the safe side use 128K.
2. the only fdisk utility on the device is sfdisk. fdisk could be extracted from Debian packages but it aligns partitions based virtual harddisk cylinders and not on flash blocks.
3. to modify the whole partition table use "sfdisk -d /dev/mmcblk0 > table", edit the file table, and overwrite the partition table with "sfdisk /dev/mmcblk0 < table"
4. ids of single partitions can be changed with "sfdisk -c /dev/mmcblk0 2 83" (this example changes the 2nd partition to 83=Linux).
5. Maemo hardcodes the MyDocs partition as /dev/mmcblk0p1 in several applications. If you keep a VFAT partition, make sure it is the 1st partition (partitions in the table do not need to have the same order as on the disk). Otherwise you have to modified several scripts belonging to ke-recv.
6. /home is the first ext3 partition, swap can be any partition number.
7. the most relevant scripts for partitioning are /etc/event.d/rcS-late, /usr/sbin/osso-usb-mass-storage-enable.sh and /usr/sbin/osso-usb-mass-storage-disable.sh
8. is not a good idea to unmount /home in a running system. umounting /home/user/MyDocs is generally safe (if all applications are closed).
9. you can boot without a /home partition (for example, by commenting out the line in rcS-late or by change the partition ids to something else than 83). Maemo will create a default home (about 10M) on NAND. Most relevant settings (wifi, desktop) are stored on NAND anyway. make sure to "rm -rf /home/user" before reboot. This trick is useful if want to change the layout of eMMC completely.
10. ext3 partitions can be grown online, but not be shrinked.
11. parted does not work on Maemo (it always crashed due to some mmap problem).
12. if you don't want a VFAT MyDocs partition, you need to create MyDocs/DCIM directory with users permission for the camera app to work proper permissions.

Solution #1: swap /home and MyDocs partition

TODO, see how to swap FAT and ext3 parition The following instructions are untested.

We assume you have the standard Nokia partition layout on your device.

Warning: the contents of MyDocs will deleted. Perform a backup!

All instructions must be executed s root (either from Terminal or via ssh, type "root" as user to become root). copy & paste is recommened (typos are dangerous). For the table file use "vi" or "cat > table" and Ctrl+D to quit.

umount /home/user/MyDocs
sfdisk -c /dev/mmcblk0 1 83 # optional (safer): change FAT to ext3 id 
mkfs.ext3 /dev/mmcblk0p1 # create ext3 on large partition
mount /dev/mmcblk0p1 /mnt # mount new /home
cp -a /home/* /mnt # copy contents of /home to large partition
umount /mnt # unmount it

create the file "table":

/dev/mmcblk0p1 : start= 56631360, size=  4194304, Id= c
/dev/mmcblk0p2 : start=       64, size= 56631296, Id=83
/dev/mmcblk0p3 : start= 60825664, size=  1572864, Id=82
/dev/mmcblk0p4 : start=        0, size=        0, Id= 0

continue with

sfdisk /dev/mmcblk0 < table # change partition table, swap p1 and p2
reboot; exit # reboot to re-read new table

after reboot

mkfs.vfat -F32 /dev/mmcblk0p1  # create VFAT on 2GB partition
mount /home/user/MyDocs # mount it

voila!

How to revert

Warning: /home must have less than 2GB usage! MyDocs will be deleted!

umount /home/user/MyDocs
sfdisk -c /dev/mmcblk0 1 83 # optional (safer): change FAT to ext3 id 
mkfs.ext3 /dev/mmcblk0p1 # create ext3 on smaller partition
mount /dev/mmcblk0p1 /mnt # mount new /home
cp -a /home/* /mnt # copy contents of /home to large partition
umount /mnt # unmount it

create the file "table" (this is the original layout):

/dev/mmcblk0p1 : start=       64, size= 56631296, Id=c
/dev/mmcblk0p2 : start= 56631360, size=  4194304, Id= 83
/dev/mmcblk0p3 : start= 60825664, size=  1572864, Id=82
/dev/mmcblk0p4 : start=        0, size=        0, Id= 0

continue with

sfdisk /dev/mmcblk0 < table # change partition table, swap p1 and p2
reboot; exit # reboot to re-read new table

after reboot

mkfs.vfat -F32 -s64 /dev/mmcblk0p1  # create VFAT on 27GB partition
mount /home/user/MyDocs # mount it

Solution #2: shrink MyDocs to add additional partitions

TODO, see Repartitioning: create extended partitions

Solution #3: arbitrary size for /home and MyDocs

TODO, see also User:Ruskie#Repartitioning

Warning: /home and MyDocs will be deleted. Backup everything. Backup all settings (to your computer or SD), reflash, restore the settings and install rootsh (no other applications!).

create your new partition table layout in the file "table". Make sure all partitions are aligned to 32K and sizes are multiples of 32K. The sectors are 512bytes, so the block counts must multiplied by 2. For example, 2GB= 4194304 blocks. Each partition should start at start+size of the previous partition. The first partition must start at 64. Make sure to keep the swap partition (the number may be changed). Initially set the id of your partition to 0 so that they are not mounted at reboot. An example layout with 2GB part1 and 27GB part2:

/dev/mmcblk0p1 : start=       64, size= 4194304 , Id=0
/dev/mmcblk0p2 : start= 4194368, size=  56631296, Id=0
/dev/mmcblk0p3 : start= 60825664, size=  1572864, Id=82
/dev/mmcblk0p4 : start=        0, size=        0, Id= 0

continue with

sfdisk /dev/mmcblk0 < table # change partition table
reboot; exit

The device creates a default /home in NAND. Now change the parition ids and create the file systems

sfdisk -c /dev/mmcblk0 1 c
mkfs.vfat -F32 -s64 /dev/mmcblk0p1
sfdisk -c /dev/mmcblk0 2 83
mkfs.ext3 /dev/mmcblk0p1
rm -rf /home/user
reboot; exit

finally restore all your backups

Solution #4: single large /home partition, loopback for USB

TODO, see how to implement USB mass storage with a loopback file

like solution #3, but don't create a MyDocs partition.

how to create a sparse 1GB loopback file:

dd if=/dev/zero of=FAT1GB bs=1M seek=1024 count=0 # small 256MB image
losetup /dev/loop0 FAT1GB
mkfs.vfat -F32 /dev/loop0
vi /etc/fstab # replace /dev/mmcblk0p2 with /dev/loop0
mount MyDocs

References

1. brainstorm and discussion
2. Repartitioning the flash