4. Media Failures
A media failure is the loss of some of stable storage.
Most disks have MTBF over 10 years
Still, if you have 10 disks ...
So shadowed disks are important
- Writes go to both copies. Handshake between Writes to avoid common failure modes (e.g. power failure)
- Service each read from one copy
To bring up a new shadow
- Copy tracks from good disk to new disk, one at a time
- A Write goes to both disks if the track has been copied
- A read goes to the good disk, until the track is copied