This is good idea but I think there are other aspects to take into account:
in case of RAID array failure you should have a copy pool to restore. You
can make file copypool but you cannot send disks off-site so easy. And how
to perform reclamation of off-site disks! So complete avoidance of tapes
will be hard.
price of the library itself is high but later price on cartridges is not.
Tape storage is still cheaper. LTO cartridge will hold 100GB uncompressed
data, with compression is more. Two 73 GB disks or one 180 GB for the same
capacity would weight over $1000.
you forget to take into account the electricity (V * A = W, W * h = $).
If you have to hold the backups for a month or a quarter calculate the
power consumption too.
It is approx. 0.5W/GB so for 4 TB you will have more than 2kW only from
disk. And if you have to keep 2 or 5 year archives ...
if disk extension is easy for you, tape is easier. Add new disk,
(re)build RAID array, define raw volume or filesystem, define TSM
pool/volume vs. checkin as scratch. And occasionally hot-swap bays will be
short - new enclosure to be delivered, cabling, maybe new adapter and even
someday new cabinet, etc.
think about growth rate of data being backed up. Disk pool does not have
overflow location.
For a small shop this might be very good solution. However data tends to
grow very fast. Calculate "what-if" scenario for buying library now and if
you buy it after a year or two. For sure you will be forced to do it.

Zlatko Krastev
IT Consultant






"Rooij, FC de" <Fred.deRooij AT CORUSGROUP DOT COM>@VM.MARIST.EDU> on 11.01.2002
09:04:45
Please respond to "ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU>
Sent by:        "ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU>
To:     ADSM-L AT VM.MARIST DOT EDU
cc:

Subject:        TSM, extending backup storage


Question:
I think about a huge diskpool without migration to tape.  Is there a
limitation on the size of the diskpools?

If not, are there things I have not thought about?

The backup-service is separated from the original data by a dedicated
building located 1 Km away from the original!
I think we have a lot of benefits:
*       quicker restores
*       no waiting for the availability of tapeunits
*       less administrative overhead
*       migration from disk --> tape
*       no reclamation
*       less processor use
*       the admininistrative window will decrease, so the backup window can
increase
*       no ATL
*       No tape-units
*       Easy exctending of storage capacity
*       ...........?

I am aware this is a strange idea, but why not?

with regards,
Fred de Rooij
Corus Information Services - Enterprise  Services
%    +31 (0) 251 492215 - Fax : +31 (0) 251 471153
Email: Fred.deRooij AT corusgroup DOT com