We have a HP ML350 G6 with a P410i RAID controller with 512MB battery cache with 7 disk of 146GB attached configured in RAID5 which gives us a total of +/- 819GB usable space.
Because we were running out of space i added another disk of 146GB to the server and extended the RAID5. I took a while to complete the extend which resulted in very slow performance.
Before the extend i had a datastore of 819GB with +/- 5GB of free space. After the extend i have a datastore of 820GB with +/- 1GB free space.
How in the hell can this be?