Sebastian Kösters wrote:
i have a question.
We are using a DAT-Streamer → /proc/scsi/scsi
Vendor: SEAGATE Model: DAT DAT72-000 Rev: A030
We can use tapes with max. 36 GB but we want to backup about
to 40 – 50 GB.
Fine. My tapes are AIT-1 35 Gbyte (marketing numbers, actually about
34 GB real capacity), and currently back up 220 Gbyte with amanda.
I heard something about a special compression to put this amount
of data on smaller tape.
Compression is one of the techniques.
Is this possible with Amanda?
Yes, that can be done easily.
First you should understand how amanda differs from almost all
the other backup managers, that do variations of "all full
backups on friday, and incrementals on the other days".
Instead you specify a dumpcycle, e.g. 1 week, indicating
that you want 1 full backup each week. And you tell how
many times you intend to run in such a cycle, e.g. 5 times
(each working day).
Amanda then tries to spread the full backups of all the entries
in the disklist (DiskListEntries = DLE's) over those 5 runs,
trying to level out the amount of work each night. Those DLE's
not due for a full backup, are done incrementally.
The benefits are:
Each day you have a backup of everything.
Each backup does about the same amount, taking more or less the
same time.
If for some reason an incremental backup takes much much more
space than normal, amanda reschedules everything automatically.
Your tape capacity is much larger than one tape. Each tape only
needs space for TotalData divided by runspercycle + space for
the incrementals.
If you insist, amanda can handle dumpcycles of 0 days (meaning
a full backup always).
Oh, yes, and you can also use compression on the data too.
There are two possibilities: hardware compression (by the tapedrive
itself) and software compression (by piping each backup image
through gzip). Hardware compression is usually enabled by default
on linux; you need to edit /etc/stinit.def to disable it by default.
Software compression can be enabled in the dumptype for each DLE.
You can even choose between "fast" and "best", and between running
gzip on the server or the client (to spread the CPU-load).
(Warning you in advance: "best" gains a few percent, but takes 2-4
times as much CPU-power.)
Because Amanda plans a lot in advance, she likes to have accurate
numbers. Because the compression ratio can vary enormously depending
on the data (jpg, or mp3 don't compress anything, but an empty oracle
database compresses very well) Amanda learns from the previous run
how well each DLE compresses. When using sofware compression amanda
can better predict how much data will fit on a tape. When using
hardware compression, Amanda cannot learn anything about compression
ratios, and you have to lie about the tapecapacity and make a wild
guess based on the kind of your data.
Search the archives for the eternal discussions about compression
and the pro/cons of hw/soft compression.
One thing to watch out for is to avoid using both hardware compression
+ software compression, because on all drives (except LTO-drives,
AFAIK) the hardware compression algorithm actually expands already
compressed data, resulting in a tapecapacity of 20-25% less than
expected.
Amanda.conf:
define tapetype DAT {
comment "DAT tape drives"
length 35000 mbytes # these numbers are not accurate
filemark 100 kbytes # but you get the idea
speed 100 kbytes
These nice round numbers are from the marketing department.
One of the benefits of running "amtapetype -e 35g" is that you
also get a warning if the hardware compression is still enabled.
And you get more accurate numbers (which you should round off to
some number below, to account for variations in tapelength between
cartridges.
OS- Version RH 9.0
Amanda: 2.4.3
Fine, that works, but notice that currently the stable amanda version
is already 2.4.4p3.
--
Paul Bijnens, Xplanation Tel +32 16 397.511
Technologielaan 21 bus 2, B-3001 Leuven, BELGIUM Fax +32 16 397.512
http://www.xplanation.com/ email: Paul.Bijnens < at > xplanation.com
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