Andrew Quintana wrote:

The problem with letting the CDL handle staging to physical tape is a
matter of tape usage and capacity.  Since the compression ratio you'll
get on physical tape is dependent on the data being written, it can't be
reliably predicted.  So when the EMC CDL creates virtual tapes it limits
them to a pre-determined size (which you can change).  By default CDLs
do not compress the data they write to the virtual tape, which is good
because of the huge performance penalty they take when compressing data.

As an example, say you have LTO-3 tapes, with 400 GB native/~800GB
compressed capacity.  If you emulate LTO-3 tapes in the CDL it will
default to 400 GB capacity, which is not compressed in any way.  When
the CDL migrates data to a directly attached physical library, it does
it on a volume by volume basis.  So your 400 GB of data on a virtual
tape will be migrated to a physical LTO-3 tape which will compress in
hardware.  You will end up filling only ~50-60% of that physical tape.
You can force the virtual tapes to larger sizes to try to compensate, at
the risk of data migration failing because a particular virtual volume's
data will not compress as well as you have predicted.  You save an
autochanger license, but use twice as much physical tape.  You save on
the licensing and support costs, but pay more for media and extra time
spent shuffling tapes.

It's a trade-off.  I prefer to run the CDL and the physical autochanger
side-by-side.  You have to license them both, but you gain the ability
to fully use your physical media by cloning/staging in Networker, which
is saveset-based.  In addition, you can always run some backups directly
to physical tape which you couldn't do if the physical autochanger was
hidden behind the CDL.

YMMV.  I can only speak from experience with CDLs, other VTLs may
differ.  I haven't gotten a chance to play with the latest CDLs yet, so
I don't know what EMC might have done regarding this issue.

Andy

-----Original Message-----
From: EMC NetWorker discussion [mailto:NETWORKER AT LISTSERV.TEMPLE DOT EDU] On
Behalf Of Stuart Whitby
Sent: Thursday, August 16, 2007 10:00 AM
To: NETWORKER AT LISTSERV.TEMPLE DOT EDU
Subject: Re: [Networker] new to Virtual Tape Library - what are the
benefits?

Depending on the VTL involved, it will manage the cloning of data to
physical tape.  If this is license-free, then it may be better to do
this than to involve NetWorker, which will require a further jukebox
license to cover this.  The way it works on a CDL is that the CDL
controls the physical library, and will load tapes and route the data to
NetWorker that way.  Or so I believe - didn't have physical libraries on
my last job :)

Cheers, Stuart.

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