"Any existing Ext3 filesystem can be migrated to Ext4 with an
easy procedure which consists in running a couple of commands in
read-only mode (described in the next section). This means that you
can improve the performance, storage limits and features of your
current filesystems without reformatting and/or reinstalling your
OS and software environment. If your need the advantages of Ext4 on
a production system, you can upgrade the filesystem. The procedure
is safe and doesn't risk your data (obviously, backup of critical
data is recommended, even if you aren't updating your filesystem
:). Ext4 will use the new data structures only on new data, the old
structures will remain untouched and it will be possible to
read/modify them when needed. This means, that, of course, that
once you convert your filesystem to Ext4 you won't be able to go
back to Ext3 again (although there's a possibility, described in
the next section, of mounting a Ext3 filesystem with Ext4 without
using the new disk format and you'll be able to mount it with Ext3
again, but you lose many of the advantages of Ext4)
"Bigger filesystem/file sizes
Currently, Ext3 support 16 TB of maximum filesystem size, and 2 TB
of maximum file size. Ext4 adds 48-bit block addressing, so it will
have 1 EB of maximum filesystem size and 16 TB of maximum file
size. 1 EB = 1,048,576 TB (1 EB = 1024 PB, 1 PB = 1024 TB, 1 TB =
1024 GB). Why 48-bit and not 64-bit? There're some limitations that
would need to be fixed before making Ext4 fully 64-bit capable,
which have not been addressed in Ext4. The Ext4 data structures
have been designed keeping this in mind, so a future update to Ext4
will implement full 64-bit support at some point. 1 EB will be
enought (really :) until that happens. (Note: The code to create
filesystems bigger than 16 TB is not in any stable release of
e2fsprogs)"
