I think you hit every nail right on the head, Milton.  I would emphasize
#5, as people tend to minimize how big of a deal it is to provision and
administer large amounts of disk.  A GOOD VTL (not all are good) will
make that provisioning issue go away.

As to de-dupe, it's considered by many to be the hottest technology
right now, and it is real, and is saving lots of people lots of money.
It actually (in most cases) makes a VTL roughly equivalent (if not
cheaper) than a similarly-sized tape solution.

---
W. Curtis Preston


-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Johnson, Milton
Sent: Friday, June 15, 2007 12:09 PM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Re: [ADSM-L] Fw: How to Incorporate a CDL into TSM environment?

Why a VTL vs FILE devclass volumes on local drives?

1) With a VTL you can do LAN free backups.

2) Data Compression:
2A) TSM Client does compression: Big performance hit on the client,
slower backups/restores
2B) TSM server does compression (FILE devclass volumes on compressed
file systems): Big performance hit on server, slower
backups/reclaims/restores
2C) VTL does compression: No performance hits (just as with using
physical drives with compression)

3) I doubt the TSM server could write large amounts of data
simultaneously streaming in from 30 different clients to 30 different
FILE devclasses volumes as fast as it can write that data to a fibre
channel adapter.
3A) 30 input streams means 30 mounted FILE devclass volumes
3B) The drive heads would always be out of position for the next write.

4) Data Reduction/Data Deduplication/Content Aware Compression: What
ever the VTL vendor calls it, you don't have that available with FILE
devclass volumes.  (I realize that from previous posts you are not
comfortable with this technology, I myself have not used it.)

5) Do you really want to manage an AIX system with a 25TB, 50TB or a
100TB file system?  After a system crash how long will a FSCK of 100Tb
take?

Thanks,
H. Milton Johnson
-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Allen S. Rout
Sent: Thursday, June 14, 2007 3:22 PM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Re: [ADSM-L] Fw: How to Incorporate a CDL into TSM environment?

>> On Wed, 13 Jun 2007 08:43:43 -0400, "Johnson, Milton"
<milton.johnson AT CITI DOT COM> said:

> The VTL is a Sepaton S2100-ES and yes it is disk only.

> I don't see the benefit that a "tape backed" system would bring, how 
> does that really differ from a physical tape ATL with TSM providing a 
> DISKPOOL front end?

Well, exactly. :)

But the distinction I wanted to make clear was: if you've decided to
store all your data on disk, then TSM has all the primitives necessary
to make that disk manageable, and you can discard the intermediate
appliance that makes the disk pretend to be a bunch of tape drives.
That's what everyone's getting at when they talk about FILE devclasses.

So if you bought 23 TB of slow disk plus a pretend-im-tape-box, then the
tape box was a waste, if you're using TSM.  If you're using something
without TSM's volume primitives, it could be extremely important.


- Allen S. Rout