On Mon, Mar 14, 2005 at 09:14:43AM -0700, Tom Schutter wrote:
> I had some questions regarding tapecycle, and after reading the man
> page and the doc (old and new), I think that they fall short on
> describing what tapecycle should be set to. The minimum value of
> tapecycle is well covered, but not the maximum value, and how
> tapecycle should relate to the number of tapes that have been
> labeled.
>
> >From the man page:
> tapecycle int
> Default: 15 tapes. The number of tapes in the
Gee, I did not realize there was a default :)
> active tape cycle. This must be at least one
> larger than the number of Amanda runs done during a
> dump cycle (see the dumpcycle parameter) times the
> number of tapes used per run (see the runtapes
> parameter).
>
> For instance, if dumpcycle is set to 14 days, one
> Amanda run is done every day (Sunday through Satur-
> day), and runtapes is set to one, then tapecycle
> must be at least 15 (14 days * one run/day * one
> tape/run + one tape).
>
> In practice, there should be several extra tapes to
> allow for schedule adjustments or disaster recov-
> ery.
>
> So what is an "active tape cycle"? That is never defined anywhere.
Bad wording. And it is seldom good practice to use a term (eg tapecycle)
in the definition of the term.
>
> Although the last sentence is correct and it makes sense, it does not
> explain how tapecycle should relate to the actual number of labeled
> tapes.
>
> Here is my bad attempt at an improvement, please do not use it verbatim:
>
> You must have at least tapecycle tapes labeled, but you can have
> more. By labeling extra tapes, you can allow for schedule
> adjustments or disaster recovery. For example, lets say that your
> tapecycle is set to 20 and you have 20 labeled tapes. If you
> discover that tape #5 that you are about to put in the drive is bad,
> your only alternative is to immediately label a new replacement
> tape. If tapecycle was 20 and you had 25 labeled tapes, then you
> could put tape #6 in the drive and deal with the problem later.
>
> On the other hand, if the number of labeled tapes greatly exceeds
> tapecycle, then AMANDA (insert inefficiency issue here).
Two things; I know of no inefficiency issues related to exceedingly
large numbers of tapes in rotation. Or other problems, except cost,
even in using fresh tapes every run. And as to your suggested
revision, in writing man page documentation one must judge how much
example, description, and definition should go into a document that
is intended to be terse and quickly readable as reference, not how-to.
Here is my attempt at a revision:
tapecycle int
Default: 15 tapes. Typically tapes are used by amanda in
an ordered rotation. The tapecycle parameter defines the
size of that rotation. The number of tapes in rotation must
be larger than the number of tapes required for a complete
dump cycle (see the dumpcycle parameter). This is calculated
by multiplying the number of amdump runs per dump cycle
(runspercycle parameter) times the number of tapes used per
run (runtapes parameter). Typically two to four times this
calculated number of tapes are in rotation.
While amanda is always willing to use a new tape in its rotation,
it refuses to reuse a tape until at least 'tapecycle' number of
other tapes have been used. It is considered good administrative
practice to set the tapecycle parameter slightly lower than the
actual number of tapes in rotation. This allows the administrator
to more easily cope with damaged or misplaced tapes or schedule
adjustments that call for slight adjustments in the rotation order.
--
Jon H. LaBadie jon AT jgcomp DOT com
JG Computing
4455 Province Line Road (609) 252-0159
Princeton, NJ 08540-4322 (609) 683-7220 (fax)
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