Linux – Running resize2fs on /

It's mounted, but it won't show as an opened file (try it with other mounted drives). Do grep "/dev/md" /proc/mounts and see if it says where it's mounted. If not, see what cat /proc/mdstat says. Once you've unmounted it, fsck should work.

You may have a failed disk, see what mdadm -D /dev/md4 says - if it gives a dodgy disk, remove it mdadm -v /dev/md4 -r /dev/whatever, replace it or fdisk it, then mdadm /dev/md4 -a /dev/whatever

The lvextend command you executed would only extend the filesystem by 800MB (assuming default extent size), a rounding error when you're looking at TB filesystems. The -l flag means "extents" which, by default, are 4MB in size. If you wish to grow the filesystem by, say, 200GB, the command would be: lvextend -L +200G vg2/lv_backup. (Note the difference between -l and -L.)

The problem is that the initial filesystem was 1TB (almost) and the new one is 800GB, so the new logical volume should now be around 1.8TB. From here, you can see that both the drives are part of the same volume groups:

The initial filesystem was 1TB. The new disk is 800GB. You added that disk to the VG, then used 800MB of that disk to extend the LV, then grow the filesystem on the LV, leaving you with 1.0008TB in the LV. Then you grew the filesystem to fill the LV.

If you want to end up with a 1.8TB filesystem, do this:

lvextend -L +800G vg2/lv_backup

or:

lvextend -l 100%FREE vg2/lv_backup

Followed by the resize2fs command.