[Info-vax] FREESPADRIFT

[Johnny Billquist]

On 2016-06-12 21:04, Phillip Helbig (undress to reply) wrote:
> What causes the free-space count to become wrong?  Since VMS obviously
> knows about it (otherwise /REPAIR couldn't fix it), why isn't it always
> correct?
>
> Is there any reason not to run ANA/DISK/REPAIR, say, once per day?

Guessing, based on RSX and ODS-1, the count is kept and updated as files 
are created/deleted/extended/truncated. However, there is no guarantee 
that nothing bad happens sometime, that is not reflected in an update in 
that count. Such as if the system crash while a file system update have 
not been flushed to disk yet. Or only partially updated.

ANALYZE, on the other hand, actually scans through all disk blocks used 
by the file headers, and then update the block used bitmap, and block 
used counts, based on this. So it is updated to become the true, correct 
count of how things actually are on disk right now.

Reasons not to run it every day? Well, that would be because you need to 
make sure the file system is not in use when you're running it, since 
you do not want updates done while ANALYZE is running. Also, it can be a 
rather CPU and I/O intensive operation to run ANALYZE. So it is 
inconvenient and disruptive to run it.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol