The major downsides would be that I wouldn't be able to use some nice tools that Linode provide, such as their automatic partition resizing and automatic backup solution, and their rescue distribution, Finnix, doesn't support it. (Edit: Their rescue distribution does now support ext4.)
Obviously you'd want to make sure you have a full backup before proceeding with this. I didn't, but I'm an idiot.
First you need to make sure you're using a recent kernel — probably 2.6.30 or higher. This is because some applications had made bad assumptions. I'm using the latest 2.6 Paravirt that Linode provides, which is 188.8.131.52-x86-_64-linode16. My system is Ubuntu 10.10 64-bit.
- From the Linode Platform Manager, restart in rescue mode.
- Run the following commands to update Finnix with a recent e2fsprogs that supports ext4:
apt-get install debian-archive-keyring e2fsprogs
- Perform the actual ext4 conversion:
tune2fs -O extents,uninit_bg,dir_index /dev/xvda(It will no longer be ext3 compatible after running this.)
e2fsck -fDC0 /dev/xvda
- Shutdown and boot up through the Linode Platform Manager
- The system should automatically mount the partition as ext4.