Migrating to EXT4

Ext4, the successor to ext3 which was formerly known as ext4dev, is marked stable in Linux kernel 2.6.28, meaning the Linux kernel team now recommends using ext4 in production.

To convert a file system from ext3 to ext4, use

tune2fs -O extents /dev/DEV

and remount the file system as ext4. (Two e2fsck runs are recommended before and after tune2fs.) Some documentations also include the -E test_fs option. This is not necessary now since ext4 is no longer experimental.

Finally do not forget to modify /etc/fstab.

An ext4 file system created this way is not a “true” ext4 - the extents feature, the main advantage of ext4 comapred to ext3, is not automatically applied to old files. New files created afterwards are in the extents format.

Unlike the 100% backward compatibility of ext3 with ext2, an ext4 file system can no longer be mounted as if it were an ext3, unless the extents feature is disabled. (If you want to disable extents, why not simply use ext3?)

Shrink an LVM partition

Gentoo’s Documentation says it is easier to increase the size of an LVM partition than to shrink it, so they recommend starting with smaller partitions and then increase them as needed.

Of course they are not saying shrinking is impossible, but only less easy. The most important thing is the FS must be reduced BEFORE the volume, or the FS will get damaged.

Example: Shrink partiton /dev/vg/home with ext3 filesystem from 30G to 12G:
e2fsck /dev/vg/home (resize2fs requires a check before resizing)
resize2fs /dev/vg/home 10G (resize FS to 10G)
lvreduce -L12G /dev/vg/home (reduce partition)
resize2fs /dev/vg/home (extend FS to the whole partition)

Someone (I’m not sure who..) recommended doing two resizes like this claiming it is safer.