HAMMER cleanup

[Thomas Nikolajsen]

>>- rc.d/hammer_cleanup
>>Executes at boot time.
>>Cleanup by default; ability to disable (rc.conf).
>>Fine for systems turned off at night.
>>
>>I see no problem by enabling cleanup in both rc.d and periodic by default,
>>as `hammer cleanup' is a no op. if executed more than once a day.
>>
>>If users want snapshot more often (eg. every hour), a cron script can do that,
>>this is very light weight to generate; normally it will normally be enough to
>>prune & reblock file system once a day.
>>
>>I can make scripts for this, when we agree on what we want. 
>
>I fully agree and had talked to Matt about this in IRC.
>
>See the patch for periodic at http://leaf.dragonflybsd.org/~swildner/hammer_periodic.diff.
>If no one objects, I'll commit within the next few days.
>
>About the rc script: I'm not sure because for a machine which was turned off for a longer time
> this (if enabled by default) would make booting take a rather long time.

My idea is to do rc.d cleanup in background (forgot to write this), it will last max 20 (4x 5) minutes
per PFS using default values for `hammer cleanup'; if you have many PFS this can be a long time.
This can be long time if many PFSs are defined.

But, if we don't make rc.d/hammer_cleanup run by default, I guess many newbie users will never
have their HAMMER file systems cleaned. Expert users can just disable this rc.d script,
or if they forget, kill cleanup job when running, it is always ok; pruning / reblocking will just happen later.

 -thomas