At 07:04 PM 6-9-98 +0100, Pete Vickers wrote:
>a customer of a colleague has just upgraded a 1503 (2gb) dat to a 1504 (8gb)
>dat.  It is still indicating a full tape at the same place.

As Steve Dirickson indicated, the 1504 uses the same mechanism as the 1503;
one which supports only 90m (and shorter) tapes, which hold up to 2GB of
data native.  The significant difference in the 1503 (a DDS-1 drive) and
the 1504 (a DDS-DC drive) is support for "hardware compression" (which is
actually implemented in firmware if it matters to anyone ;).  If the data
being written to the tape is already being LZ compressed (including the
high compression modes of RoadRunner, TurboStore and most other competitive
backup products) or (less likely) the DCLZ compression is disabled on the
drive, then there won't be any improvement in the tape used.  And if since
the same mechanism is used, there won't be any improvement in speed either;
both drives read and write at a maximum speed of 183KB/S.

Upgrading to a DDS-2 or DDS-3 drive will speed up the backup, but still
won't make any additional data fit on the tape without changing to higher
capacity media such as the 120m 4GB DDS-2 tapes or 125m 12GB DDS-3 tapes.
(Note that use of media with a capacity higher than the drive is
unsupported; and in this case won't work.)  That is to say that the higher
densities supported by DDS-2 and DDS-3 drives is only used on media which
supports it.  Therefore, if the tape is really a DDS-2 drive (contrary to
your claim it's a 1504 but implied by your mention of 8GB estimated
compressed capacity) it can only store that much data if they upgrade to
120m media.

It's common these days for tape vendors to quote a 2:1 compression ratio
implied in the capacity of tape drives which ship with support for
compression, even software compression in many cases, so comparing the
capacities with existing non-compressing drives used with third-party
software compression is less than obvious until you convert everything to
native (uncompressed) capacity or to compressed capacity.  The capacities I
used above are native (uncompressed) capacities.  The actual compression
achieved is dependant on the data being compressed and the algorithm used.

Overall on most systems 2:1 compression is typical, but many data files
(including typical Image databases in my experience) often achieve roughly
a 4:1 compression ratio, so it's not uncommon on the 3000 to see 4:1
compression quoted.  I have personally seen full backups of production
systems which achieve better than 10:1 compression and others which achieve
less than 2:1 compression.  In this case, "your mileage *will* vary".
--
Jeff Woods
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