I'm told that we have a RAID5 diskset. I am told that its capacity is equivalent to the capacity of the disks in the set minus one disk, which we can consider to be the capacity used for parity (and we ought to have minus another disk, which is reserved for hot swap, but let's ignore that).
I am told that the storage required for 1GB of data is a bit more than 1GB, but not as much as 2GB (1GB plus the parity bits - we can ignore all of the file system stuff, because my files are very large compressed data).
I am also told that if a catastrophe hit one of those disks, my 1G of data would not be lost.
Have I been misled?