[KLUG Members] Question on RAID - 1 ( Number of partitions)

[Sanjay Chigurupati]

Hi,
I am trying to create a software RAID ( RAID 1) with 2 hard disks.  I
created two software raid partitions for each mount point I wanted.

For instance for /home, I created two software raid partitions (one on hard
disk 1 and one on hard disk 2) and created a raid device using those two
partitions.

I kept the number of spare disks as zero.

When I removed a hard drive from the machine (disconnected, with power off)
and restarted, I was able to get into the os.
But when I reconnected the hard drive, and ran /sbin/hotaddraid  /dev/md1
/dev/hdc

It was not able to add the second hard drive back. It said :
md1 : unable to reconstruct. no spare disk
running in degraded mode

What is purpose of spare disk? If I use it, it looks like I will need three
software raid partitions for each device. Is this right?

I was under the impression that with RAID 1, if I used 2   40 GB hard
disks, my total data could use 40 GB. With spare disk, it looks like it
will be 1/3 of 80.

Is it possible to recover without spare disk.

Thanks,
Sanjay