Are /usr and /var for this setup really larger than on my phone???
Did you somehow reverse the optifying to use /usr?

Quick reply...
I tried to build kiconedit on N900 itself.
So: I installed subversion, got the source, installed gcc, said "gcc main.cpp", found out that it needs general KDE headers, started copying them one-by-one.
It turned out that kdemacros.h should be generated by cmake. So I copied kdemacros.h.cmake and said
apt-get install cmake

It installed cmake-data and then couldn't find space for cmake

I manually optified cmake-data (if I understand anything in this filesystem at all )

There still wasn't enough space for cmake

Almost all languages in /usr/share/locale were already deleted earlier.

So I thought: what about optifying /usr/lib?

I moved all files from /opt/usr/lib to /usr/lib and then all files from /usr/lib to /opt/usr/lib.
But... /usr/lib/python2.5 and two more folders which actually already are in /opt/pymaemo continued to be mounted in /usr/lib. I wanted to mount them in /opt/usr/lib.
After some mount umount -l bind move.
Mess increased, but /usr/lib empty, and then replaced with symbolic link to /opt/usr/lib

For some time, everything worked, including apt-get

But later, MicroB couldn't open any page at all

I took a wild guess that after reboot nothing will work (why???)

So I backup-ed some most important files (like Hildon menu; I put a lot of effort into customising it with sub-menus) and, just in case, moved photographs tp PC

Reboot
Malfunction
Re-flash
Home isn't harmed, but all applications are gone

Now, my ambitious goal is:
to have all applications optified to the extent that less than half of rootfs is usually used

How can I reach it?

Now 60% are in use.
I have Modrana (and I don't have OSM logger and Marble and OSMapper, and I'm not going to install them)
I have Panorama and MBarcode (and I don't have FrontView, and I'm not going to install it)
I have Fennec and Chromium
I'm going to get Metre and QStarDict
I'm going to get eboard, sudoku and mokomaze (probably; not sure yet)
I'm not going to install Maelophone (I don't think that emulating physical music on an electronic device is worthwhile goal)
I'm not going to install my own half-baked programs: GPS compass and auto-rotation and text editor
I'm not going to install MyPaint, PenPen, and example Qt Text Editor
I have already installed pocketsphinx, and voximp wasn't lost while reflashing
I have tried to install ejpi... but purged it. It's awkward, though innovative.
I'm trying to get aptitude GUI working - then I will not need fapman
I will have to add repositories to HAM again - they were lost.

@michaaa62 :
This is what I have
In my setting, I changed /opt from /home/opt to /usr/opt (700MB) but even without moving /opt the size is more than rootfs can support. I am reverting to classic /home/opt as my phone is often going into infinite reboot when mounting /opt in /usr.

I really need helppp.....My phone rebooted itself when i was doing the solution #1.
: cp -a /home/* /mnt # copy contents of /home to large partition.
i have tried to restore the entire system but i cant access the MyDoc in File Manager after restoring.

Well, you just had it re-formatted according to the howto, so no wonder it is not there. Go on with copying all of the content of /home to /mnt (after a reboot you have to mount the partition /dev/mmcblk0p1 to /mnt again !!! ) to be sure the copy process was not interrupted.

but it says not found when mounting plz help

Could you pls give the link to gparted on n900.. I did some search on the forum and else where. Could not find it except for this

@ babieshame:
Short Walkthrough:
This removes MyDocs from the file system, but still all the data on partition /dev/mmcblk0p1 available.
This changes the file sytem type in the partition table from Fat32 to Linux. Still all the data there, but you would get an error after boot.
This wipes the filesystem and the data from the partition, you would stiill be able to recover with extra effort, like testdisk data recovery tool.
This will make the partition usable for the system again, but will not access your data, but make it possible to overwrite it.
This actually overwrites the data with the stuff from your optfs and the content of /home, but only the empty folder MyDocs is copied!
This unmounts the partition formerly known as MyDocs now containing the optfs.

The howto continous to create a new partition table, here called 'table' for simplicity to get the partitions mounted for the next reboot. After you wrote the table to the device, you are ready to reboot. After the reboot you simply create and then mount the new file system for the now smaller MyDocs partition.

Because you first asked about the cp command i guess there is no MyDocs existing any more. Continue like suggestedin the post above with the howto from the "mount /dev/mmcblk0p1 /mnt"-step to achieve a working setup. right now you are in the middle of nowhere.

parted is not gparted!!! The gparted howto works from a linux desktop/laptop only.

It is in the repository (version 2.3-1) but it isn't gparted.
For N900 exists only parted (console version)