Hp SureStore DAT8

I have an HP Surestore DAT8. HP number is C1533A. This is supposed
to back up 4 Gig native and 8 Gig compressed. Now I have about 7.7 G
native on my hard drive to backup. So obviously I need to use
compression to fit it all on one tape. Now the HP backup device is
reporting 4.9 Gigs of data compressed. It should all fit on one tape
since I have not exceeded the compressed capacity of the drive. The
drive should do 4 native and 8 compressed but at about 4.7 Gigs
compressed it asks for another tape. I check with HP tape tools to
see if there is anything wrong with the compression mechanism and it
reported no errors. So I was wondering, is there something I am
overlooking or is there something I don't understand about compression
ratios? What I would like to do is fit my 7.7Gigs of uncompressed
data on one tape using compression.

schrieb:
> It should all fit on one tape
> since I have not exceeded the compressed capacity of the drive. The
> drive should do 4 native and 8 compressed but at about 4.7 Gigs
> compressed it asks for another tape.

The specified compression ratio is an average value.
If your data isn't compressible, bad luck for you.
The 2:1 compression ratio is achievable only for
highly redundant data, i.e. plain text with lots of spaces
or binary data with lots of zeros. Jpegs and such are already
compressed, so the drive can't top that.

On Sun, 30 Dec 2007 08:31:26 -0800 (PST),
wrote:

> I have an HP Surestore DAT8. HP number is C1533A. This is supposed
> to back up 4 Gig native and 8 Gig compressed. Now I have about 7.7 G
> native on my hard drive to backup. So obviously I need to use
> compression to fit it all on one tape.

You need compression to get more than 4GB onto the tape. I have
several similar drives (C1552b) and possibly a C1533A hiding
somewhere, and using HP's hardware compression often got about 6GB
of data on each tape.


> Now the HP backup device is reporting 4.9 Gigs of data compressed.

I assume that you mean that the backup software is reporting
4.9GB. (I've never used HP's backup software or tools.) Are you
sure that you fully understand what the software is trying to tell
you? It *might* be that it's saying that based on it's assumed
compressibility factor, it will require two tapes to hold your
backup set. The first, which will hold slightly less than 4GB, and
another tape to hold the remaining 1GB (approx.) of compressed data
that it expects to find when the backup is actually performed.


> It should all fit on one tape since I have not exceeded the compressed
> capacity of the drive. The drive should do 4 native and 8 compressed
> but at about 4.7 Gigs compressed it asks for another tape.

Then feed it another tape! Most of my backup data sets
required more than one tape, and I used several ways to backup the
data. The simplest was to just feed more tapes as requested by the
software, creating one complete backup. From that point on, the
backups (if I decided to only backup new or changed data) would
either be incremental or differential backups, and each of these
backups used a very small fraction of a DAT tape, so I could often
get nearly an additional 10 backups per tape. This complicates tape
restores, so I most often did only complete backups, but defined
data subsets, either in the backup software's rules, or physically,
by partitioning a large hard drive into volumes ranging in size from
1.9GB to about 3.9 GB. This pretty much guaranteed that
impressibility wasn't a factor, and a tape would always be able to
completely backup a couple of the smaller disk volumes or one of the
larger ones.

Much better than 4GB DAT drives in several ways would be cheap
DVD-RAM drives. The discs have slightly greater capacity, the media
is cheaper, and unless you decide to do incremental (as opposed to
differential) backups, won't even need backup software for backups
and restores. It's a bit riskier than tape, in that the DVD-RAM
disks resemble hard drives, so you may be tempted to run software
directly from them. This might allow some of the 'backed up' data
to be modified, ruining the integrity of the backed up data.
Backing up and restoring files can be done using Windows Explorer,
or by any other means you have of copying or transferring files. I
don't know who actually made my DVD-RAM drive, but it's the one
supplied by HP with the computer.

* :

> I have an HP Surestore DAT8. HP number is C1533A. This is supposed
> to back up 4 Gig native and 8 Gig compressed.

Right. But be aware that the compressed size is just a (very
optimistic!) average and not a fixed given capacity.

> Now I have about 7.7 G
> native on my hard drive to backup. So obviously I need to use
> compression to fit it all on one tape. Now the HP backup device is
> reporting 4.9 Gigs of data compressed. It should all fit on one tape
> since I have not exceeded the compressed capacity of the drive. The
> drive should do 4 native and 8 compressed but at about 4.7 Gigs
> compressed it asks for another tape.

Right, and that's fully ok.

> I check with HP tape tools to
> see if there is anything wrong with the compression mechanism and it
> reported no errors. So I was wondering, is there something I am
> overlooking or is there something I don't understand about compression
> ratios?

Yes. As stated above the 8GB compressed is not a fixed value. In fact,
there's hardly a way to get 8GB of data on the tape if not using data
that is very good compressible.

Besides that, the real-world capacity also depends on the type of data
you want to backup. Does it inlcude compressed archives (i.e. ZIP, RAR),
audio files (mp3, ogg etc) and video files (mpeg, mp4 etc)? These data
types are already compressed and thus further compression doesn't lead
to a reduction in size. In fact, it often happens that compressed files
that are backed up with tape drives with internal hardware compression
that these data use more space when being compressed again than in their
original state.

> What I would like to do is fit my 7.7Gigs of uncompressed
> data on one tape using compression.

Very likely you will be unable to do that. But then, DDS2 cartridges are
dirt cheap today.

Benjamin

On Dec 31 2007, 4:51*am, Benjamin Gawert <bgaw...@gmx.de> wrote:
> * william_ry...@yahoo.ca:
>
> > I have anHPSurestoreDAT8. *HPnumber is C1533A. *This is supposed
> > to back up 4 Gig native and 8 Gig compressed.
>
> Right. But be aware that the compressed size is just a (very
> optimistic!) average and not a fixed given capacity.
>
> > Now I have about 7.7 G
> > native on my hard drive to backup. *So obviously I need to use
> > compression to fit it all on one tape. Now theHPbackup device is
> > reporting 4.9 Gigs of data compressed. It should all fit on one tape
> > since I have not exceeded the compressed capacity of the drive. *The
> > drive should do 4 native and 8 compressed but at about 4.7 Gigs
> > compressed it asks for another tape.
>
> Right, and that's fully ok.
>
> > I check withHPtape tools to
> > see if there is anything wrong with the compression mechanism and it
> > reported no errors. *So I was wondering, is there something I am
> > overlooking *or is there something I don't understand about compression
> > ratios?
>
> Yes. As stated above the 8GB compressed is not a fixed value. In fact,
> there's hardly a way to get 8GB of data on the tape if not using data
> that is very good compressible.
>
> Besides that, the real-world capacity also depends on the type of data
> you want to backup. Does it inlcude compressed archives (i.e. ZIP, RAR),
> audio files (mp3, ogg etc) and video files (mpeg, mp4 etc)? These data
> types are already compressed and thus further compression doesn't lead
> to a reduction in size. In fact, it often happens that compressed files
> that are backed up with tape drives with internal hardware compression
> that these data use more space when being compressed again than in their
> original state.
>
> > What I would like to do is fit my 7.7Gigs of uncompressed
> > data on one tape using compression.
>
> Very likely you will be unable to do that. But then, DDS2 cartridges are
> dirt cheap today.
>
> Benjamin


I still don't get it. If I have 7.7GB of native date and after
compression I have 4.9GB of date why doesn't it all fit on a 4/8 DDS-2
tape. If I can fit 8GB of compressed data and I only have 4.9GB then
I should be able to do it all on one tape.

On Tue, 01 Jan 2008 17:11:24 +0100, <> wrote:


>
> I still don't get it. If I have 7.7GB of native date and after
> compression I have 4.9GB of date why doesn't it all fit on a 4/8 DDS-2
> tape. If I can fit 8GB of compressed data and I only have 4.9GB then
> I should be able to do it all on one tape.

because compression cannot occur 2 times, one by software (7.7 -> 4.9),
one by hardware (4.9 -> 4)
4GB native, 8GB at best.
as already told, a database is highly compressible, jpg-avi-mgp-mp3-tiff
are not.

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m'écrire *
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* :

> I still don't get it. If I have 7.7GB of native date and after
> compression I have 4.9GB of date why doesn't it all fit on a 4/8 DDS-2
> tape.

Data can't be compressed ad infinitum. Data that is already there in
compressed format (i.e. mp3, jpg, mp4, ogg etc) can't be compressed any
further.

> If I can fit 8GB of compressed data and I only have 4.9GB then
> I should be able to do it all on one tape.

Please read again what others and I have answered. The 8GB mark is a
theoretical value that can't be reached with real-world data, period.

Benjamin

wrote:
[...]
> I still don't get it. If I have 7.7GB of native date and after
> compression I have 4.9GB of date why doesn't it all fit on a 4/8 DDS-2
> tape. If I can fit 8GB of compressed data and I only have 4.9GB then
> I should be able to do it all on one tape.

Don't worry! To many people compression is a miracle. The point is, it
*isn't*.

The *physical* capacity of the *tape* can and does not change. It is
4GB. I.e. the *tape* can hold 4GB. So if *after* compression, the
*resulting* (compressed) data is more than 4GB, it can and will not fit.

The *problem* is that beforehand you don't know how compressable your
data is. You can get an *indication* by trying to compress it with some
software utility, but whatever that software utility is, it won't use
the *exact same* compression algorithm as the hardware/firmware in the
DDS drive is using. I.e. the hardware/firmware compression may be better
or worse.

You didn't mention a (hardware/software) platform. For some/most
platforms, there are utilities which can report what the actual achieved
compression is. That does not help 100%, because it can and does only
report what *has been* achieved, i.e. also those utilities can not
predict the compressability of *future* data.

I hope this helps.

--
Frank "Support person for this stuff in a previous live." Slootweg