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

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

[Fernan Bolando]

Hi

The p9p vac discussion made me re-think the way I use vac.

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?

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?


fernan

--
http://www.fernski.com

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

[Nathaniel W Filardo]

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On Mon, Jun 29, 2009 at 09:13:09AM +0800, Fernan Bolando wrote:
> Why is -q not a default? Is there a reliability concern with that option?

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.

--nwf;

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

[erik quanstrom]

> On Mon, Jun 29, 2009 at 09:13:09AM +0800, Fernan Bolando wrote:
> > Why is -q not a default? Is there a reliability concern with that option?
>
> 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.

one would think that 20 bytes per file + fixed buffer would be enough.

- erik

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

[Russ Cox]

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

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

[Nathaniel W Filardo]

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On Mon, Jun 29, 2009 at 08:56:06AM -0700, Russ Cox wrote:
> 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.

You're right; I glitched.  The memory consumption is due to -d, not -q, of
course.  Sorry for the noise.
--nwf;

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