Growing SCSI defects list

[raymond.meyer%rambler.ru@localhost]

One of my scsi disks has a constantly increasing "grown defects list",
i.e. every month or so, the list grows. At the moment:

Primary defects list has 2565 defects
Grown defects list has 174 defects <- this list keeps increasing

Is this normal, or does this mean my disk is failing and needs
replacing asap?

Also, how does it detect and manage new defects? What happens to the
data on defective sectors, does it silently get truncated to 0s, or is
there some kind of failsafe way to remap the sectors and keep the data
intact? If so, what is the limit on how many defects can be fixed this
way, before I start losing data?