Re: [9fans] when to use vac -q -d old.vac instead of simply vac -d old.vac

[Russ Cox <rsc@swtch.com>]

On Sun, Jun 28, 2009 at 6:13 PM, Fernan Bolando<fernanbolando@mailc.net> wrote:
> man vac
> "-q    Increase the performance of the -a or -d options by detecting
> unchanged files based on a
>        match of the files name and other meta data, rather than
> examining the contents of the files"
>
> Why is -q not a default? Is there a reliability concern with that option?

If the file contents change but the mtime and size remain
the same, then vac -q will not notice the change and will
not back up the new file contents.  Some people worry
about this case, others don't.  Hence the flag.

> I am currently doing an hourly backup using
>
> vac -d old_date-time.vac -f new_date-time.vac /home
> which gives me a collection files with a date-time.vac filename.
>
> I am thinking I should just use vac -a main.vac /home
> to switch to this method I only need to rename latest date-time.vac to main.vac
> and delete the other ones, right?

vac -a creates a tree inside the vac archive.
It expects the archive to have a top-level
directory 2009 and subdirectories 0627, 0628, etc.
You would need to change your vac tree to have
that top-level structure before it would be
valid input to vac -a.  If you run it multiple
times per day, the subdirectories for today
would be named 0629, 0629.1, 0629.2, 0629.3,
and so on.  You can do this by building a
local file tree with the right structure and using
vac -m.

On Sun, Jun 28, 2009 at 10:02 PM, Nathaniel W Filardo<nwf@cs.jhu.edu> wrote:
> It uses an astronomically large amount of memory, if nothing else.
> Mirroring a little over 100MB of data from sources with vac -q occupies
> roughly 85MB in core.

Whether you use -q should have no effect on the memory usage.
There may be a memory leak somewhere involving -q, but at
first glance I don't see one.  Feel free to investigate.

Russ