On Wed, Jan 19, 2005 at 04:13:36PM -0500, Vicki Stanfield wrote:
> On Wed, 2005-01-19 at 10:38 -0500, Jon LaBadie wrote:
> > On Wed, Jan 19, 2005 at 09:57:22AM -0500, Vicki Stanfield wrote:
> > > 
> > > Thanks. I was unsure of whether this was a potential problem or not. As
> > > long as Amanda tells me that there was a problem and dumps to the spool,
> > > I am happy.
> > 
> > Make sure your "holding disk" space is sufficiently large and
> > that your "reserve" parameter is set to some value smaller
> > than the default 100%.  The reserve aspect is to reserve the
> > holding disk for incrementals only when there are tape drive
> > problems.  If your holding disk is sufficiently large, you
> > can also hold some full backups so the reserve should be
> > decreased.  I have enough holding disk for several days
> > of normal dumps (> 1wk) and use a reserve of 20%.
> > 
> > jl
> 
> I'm not sure that I correctly understand the reserve parameter. I have
> heard that it is used to get amanda to degrade full dumps to merely
> incremental dumps if the holding disk gets full. I assume the percentage
> refers to how full. So how does degrading to incrementals help if the
> holding disk is 100% full? What am I misunderstanding here? I can
> understand saying that if the holding disk is 75% full, switch to
> incrementals though.
> 

Definite misunderstanding.
I'll try to relate it as I understand.

Amanda can back up each DLE directly to tape with no holding disk (HD)
in use.  In that case it can only do a single DLE at a time.

With a HD it can be taping one from the HD and collecting another
onto the HD.  In fact, it can be collecting many onto the HD at the
same time.

What happens if the tape drive is broken, contains the wrong tape,
or maybe no tape at all.  Then all those DLE's collect on the HD.
I have a 6 tape changer that I swap tapes from over the weekend.
Occassionally I forget and leave for a 5 day business trip.  Now
the HD collects lots of DLE's while I'm gone.

Level 0 dumps tend to be quite large and can quickly eat up the HD.
Then what, no dumps at all?  None to tape and none to HD either?
Then I have no protection.

To reduce the likelyhood of this happening the reserve parameter
says "reserve this percent of the HD for just such a situation".
When amanda has no tape to write to and the percent of HD left
drops to this level, amanda goes into "degraded" mode and ONLY
does incrementals, no level 0's.  This keeps the backups going
to the HD a lot longer than a mixture of level 0's and incrementals.

The default reserve is 100%, i.e. no level 0's get the buffering
of the HD.  They always go straight to tape.  That is good if your
HD is small and even one or two level 0's would fill it.  But most
of us have pretty substantial HD, mine is about 100GB with a daily
taping of about 8GB.  So I can use the buffering of the HD for
level 0's and even store a good number of complete backups on the
HD before I hit my reserve setting of 20%.  That is good for my
week long trips without changing the tapes.  Then I still have
about 20GB for incremental only, degraded mode if I'm away a
really long time.  (Or like now have tape drive problems)

jl
-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
 JG Computing
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