Design challenges in chunkd self-checking

[Pete Zaitcev <zaitcev@redhat.com>]

I'm looking into adding self-checking to chunkd. This involves basically
a process that re-reads everything stored in the chunkserver and verifies
that it's still ok. Nothing can be simpler, right?

So, current problems for which I'd like input are:

 - Scheduling and deconflicting with normal operation.

   Run "genisofs" in your Fedora desktop and your Firefox is DEAD.
   It is also the reason why everyone does rpm -e mlocate the first thing
   after the installation. The effect of massive data access blowing
   away caches is very drastic in a regular Linux.
   So, I have to have a good way to keep self-checkig from interfering
   with normal service of a chunkserver.
   Also, need to save power instead of burning it on re-reading data.

 - Consistency.

   Returning wrong checksums for an object that is being updated may
   lead to us deciding to drop a perfectly good object, which is
   unacceptable (especially when redundancy is impaired already).
   So, I need some kind of locking, or logging, or invalidation...

-- Pete