Hi,
Thanks to belonis, bevanb, grorge, jreed, klarsen, lbr and many many more.
I have selected only 2 answers which I think is the best to answer to my
question. If you need the rest of the answers, feel free to email me. Once
again, thanks for help.
Original Question:-
As I am new to Digital UNIX, pls take time to read the following:-
I have a tape drive which has the capcity of backing up 8GB(uncompress) and
16GB(compress) of data using 4mm DAT tape.
However, I noticed that the backup always go up to 10.5GB, can anyone enlighten
me on this?? The backup script did not use the -C option.
By the way, can anyone also tell me what is the different between nrmt0h,
nrmt0l, nrmt0m??
Thanks in advance.
Answers :-
1.Vendor marketing nearly always assumes that data will compress
at 2:1 for pushing their capacity with compression. The truth
is the compression ratio varies with the data. Some may compress
better than 2:1, some worse, some not at all, some data can get
larger. It appears that you got some compression, but not 2:1.
You can cross check how well your data compresses by using different
compression engines. Most backup commands support writing the data
to stdout so that it can be piped to a different program:
vdump 0f - /file-system | wc -c
vdump 0f - /file-system | compress -v | wc -c
The first of these will count the number of bytes normally, the
second will compress it and show the compression ratio. If you
get a compression ratio consistent with that of the tape you
know drive is doing reasonably well.
Digital UNIX currently support four tape drive densities. Each
trailing letter of the name allows selecting a different density.
This depends on the particular tape drive supporting that many
as well. DAT drives for example only support two; with compression
and without. The variety of capacity available comes from the
choice of different length tapes. A DDS-3 drive should support
using DDS-3, DDS-2 and DDS-1 tapes with the attendant change of
capacity.
Other tape drives do support multiple densities. The TZ87, a
DLT drive, supports writing four formats:
TZ85 - 2.something GB
TZ86 - 6 GB
TZ87 - 10 GB
TZ87 w/compression - 20 GB at 2:1 compression
In this case all four names are needed. The tz manual page
has a tape of the support drives and the mapping of density
selection to name.
2. My best answer is to say : read the man page for SCSI (all caps) and go
to the section about tapes. But, the quick method is :
h = High Density (fits most on a tape)
l = Low Density
m = Medium Density
a = Auxiliary (Not sure, but maybe used for something like
a tape robot / autoloader???)
Received on Fri Aug 28 1998 - 23:42:38 NZST