Inaccurate reported available disk space

Question

I have an Ubuntu server with SAN attached storage. It has a size of 770G, using 197M and reports only 731G free space.

1. Where did the rest of the free space go?
2. Is there any way to reclaim it without formatting?

df -hl
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/data      770G  197M  731G   1% /data/mysql

Answer 1

It's probably reserved for root - mkfs.ext3 does this (reserve 5% of blocks) with default settings.

Try this if you're using ext3:

tune2fs -m 0 /dev/mapper/data

For more info and an explanation on this default behaviour see man tune2fs.