Much of the documentation out there will tell you that the benefit of the VTL 
is the speed. It is true the VTL is very fast. Some of that same documentation 
talks about not turning on the virtual compression because it will slow the 
speed. I've seen in cut the speed in half.
  But...
  I've seen a VTL with Virtualized compression turned on still operate as fast 
as real tape.  I make this point because I think you should use the virtualized 
compression. This way the same 5 TB's of disk space you were using for your 
file device class might yield 10 to 15 TB's or more of backup space under the 
VTL.  True you could turn on client side compression. But, I like the 
compression being done on the back end so that there is no stress put on the 
servers that are backing up themselves.
   
  One thing I should also clear up. In my last post I mentioned IBM with SATA 
disk. I got a friendly reminder from EMC that the CDL's have been shipping with 
SATA disk since this past November.
   
  That also reminded me that the IBM VTL called the TS7510 is using the IBM DS 
line of disk which has been out for some time now.  I remember EMC making the 
same note when it first came out with the CDL. See the disk subsystem's under 
both the IBM and EMC VTLs have been out for some time.  So just like EMC 
correctly noted when the CDL first came out you should note today about the IBM 
TS7510.  They really are not new products when it comes to the disk subsystem.  
In both cases you could choose to purchase the disk subsystems used by the VTLs 
directly from either IBM or EMC and use them with a file device class.  Granted 
I realize that both EMC and IBM have a specific configuration of their disk 
subsystems that they put under their VTLs. 
   
  In my own experience I've used a file device class with TSM V5.2 and earlier 
and an EMC CDL. I liked the CDL a great deal.  We had the same class of EMC 
disk behind a clarion setup to use a file device class.  The same amount of 
disk behind the CDL performed better.  I believe part of the reason is the 
logic in the FalconStor software. It uses disk for its virtual tapes in 5 GB 
increments and uses logic to ensure it picks the least busy disk for the next 5 
GB that is used.
   
  I know with V5.3 giving you the ability to write to multiple filespaces which 
could be on multiple LUNS gives you something over V5.2.  I still think that 
cycling through separate LUNS though isn't as good as the way the VTL allocates 
in 5 GB chunks across many more LUNS.  FalconStor may have a white paper on how 
they do it but I would encourage you to ask your vendor who ever it is to come 
on site and discuss this with you in greater detail.
   
  Whether you pick a VTL from EMC or IBM (or someone else for that matter), or 
you pick a disk subsystem with the file device class you must test yourself to 
see what will work best in your environment.  I make no claim that a VTL is for 
everyone or that it will outperform real tape in every situation.  I simply 
think it should be one of the things you strongly consider.  More and  more of 
us are seeing the benefit of moving small files off tape to disk but we may 
have been seeing 2:1 or 3:1 compression with those small files on tape. A 1:1 
of disk can be costly but when you use virtualized hardware compression behind 
a CDL it may make things more cost effective if you get 2:1 or 3:1 for small 
files.
   
  

"Allen S. Rout" <asr AT UFL DOT EDU> wrote:
  >> On Wed, 4 Jan 2006 14:42:29 -0600, "Dearman, Richard" said:

> Anyone out there have any good or bad experiences with VTL solutions. I
> was thinking about budgeting for 1 or 2 in order to phase out the
> current san file system I am using for TSM disk storage. There are
> several vendors out there with VTL solutions most notably IBM and EMC.
> My first choice would be to choose IBM but it is a new product and EMC
> has been in the market longer.

Would you be willing to expound on why you'd prefer sticking disk
behind a VTL volume virtualizer, instead of sticking it behind a
DEVCLASS=FILE volume virtualizer? I would default in the other
direction, so I'm interested in your thinking.


- Allen S. Rout
  


                
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