[Desire HD Android Development] woti23:
it is not extra ram (that would be swap or zram ie) nor has it to do with the kernel (exept the kernel must either understand ext3 or ext4, which it does anyway) but with how hboot divides the internal storage in /system /data /cache /misc /boot /recovery

/sd-ext is a mount point in rootfs. it was used on ie the original desire which had much less internal nand storage to mount an ext3 or ext4 partition there (created as a second partition on sdcard after the first fat32 partition), and to move apps (apk's not data) from /data/app (and/or/partial /system/app) to sdcard and symlink the moved apps to internal storage. in the early days called a2sd - and if additional moved dalvik-cache to sdcard a2sd+ ; although this left the /data/data on internal storage but free'd up storage, otherwise some sense roms would have run out of internal space at this time.

a difficile extention of the concept was data2sd etc ... where the data partion was a mounted partition from sdcard instead of the partition from internal storage.

the original apps2sd from android, which stored some apps on vfat partition on sdcard was seen as buggy and slow than and usb connection with cable disconnected the fat32 partition which made the programs that where on fat32 not available.

there were many variants and combinations available - as with phantasy, mount, symlinks ... (if the kernel understands even overlayed mounts or unionfs or aufs) you can do pretty much.

the data2sd frees the most storage from internal as it moves additional /data/data (the databases, settings, cache etc of the apks) to the sdcard, but is a quite more complicated issue in the partitioning scheme of the internal storage (and requires s-off).

that said this is NO solution for the poster who has 120MB internal storage free and cannot install apps. this is most likely an other issue like to small cache partition or /cache too full with leftovers or insufficient write permissions.

for example you could free up 32MB from system partition by moving /system/media/weather to ext4 partition on sdcard. or symlink some of the 187MB /system/app or 122MB /system/lib

that would not make that much sense except the dev of the rom runs out of space fitting the rom to system.

so the way for those people who need more space on the dhd would be to free up space on the data partition. i guess this may be a minority as the data partition has 1.1GB (DHD) but for some it would be an option

almost all of the /data part of the storage space is used in

/data/app = your installed apk's, so it depends on how many and what kind of apps are installed

/data/data = contains settings, cache databases, some specialized libraries and other data - size depends like above

/data/dalvik-cache = cache of extracts of apks in /system/app and /data/app, automatically recreated when wiping dalvik-cache in recovery and partitially created when installing / updating single apps

if the cache partition is big enough, /data/dalvik-cache theoretically could also be a symlink to /cache/dalvik-cache but filling up the cache partition too much may get you in trouble when downloading from market

just for the DHD

internal storage is divided in

partition mountpoint size

mmcblk0 p25 /system 550MB

mmcblk0 p26 /data 1.1GB

mmcblk0 p27 /cache 295MB

mmcblk0 p28 /devlog 20MB

+ boot and recovery partition which have no mountpoint and filesystem in normal operation

boot and recovery images are written direct to the partition - so formatting for example boot partition does not really make sense as everything is overwritten anyway.