What Nicholas says is totally true.  TSM can get very resource
constrained when it has lots and lots of simultaneous mounts to manage. 
In our design, we have a separate TSM instance running on the same
server as some of the others (AIX environment) and that instance is just
a library master, handling tape mount requests for all the others.

This instance sometimes has 170-190 simultaneous tape mounts while
servicing 10 TSM instances' tape mount requests.  We ended up having to
add "LIBSHRTIMEOUT 60" to the dsmserv.opt, too, to keep timeouts between
the library master and library clients from happening.

If I had the choice, we would have sufficient disk space in front of the
VTL for 24-48 hours worth of backups, then migrate daily to the VTL with
a much smaller number of tape mounts.  But management thought it was a
waste of money to have disk in front of disk, and at the time I had no
strong argument to support it.

On the other hand, we have some smaller environments with Windows 2003
TSM servers and 60-150 clients each, and they do their backups straight
to VTL, and we schedule them so they only have 20-30 simultaneous mounts
at a time, and this works fine.  So it depends on the scale of problem
you are trying to solve.

Best Regards,

John D. Schneider
The Computer Coaching Community, LLC
Office: (314) 635-5424
Toll Free: (866) 796-9226
Cell: (314) 750-8721


-------- Original Message --------
Subject: Re: [ADSM-L] VTL Tape Size
From: Nicholas Rodolfich <NRodolfich AT CMAONTHEWEB DOT COM>
Date: Tue, July 07, 2009 12:20 pm
To: ADSM-L AT VM.MARIST DOT EDU

Andy,

Just an opinion here. If you try to provide virtual sequential access
mount
points for each client session you may need during client processing,
you
will likely need much more system resources to complete nightly backups.
Managing mount points need only be done during daily server maintenance
processing. I would still be using a random access disk pool to provide
staging space for client backups. The L in VTL stands for Library and it
should be treated as such. For many years IBM has recommended that we
back
up to a staging area to enhance client performance and reduce resource
needs. During server maintenance, data should be placed onto longer term
storage devices (libraries) for daily expiration and reclamation
processing.. I don't think that strategy changes with a VTL.

On another not I have a client with a VTL running in a Windows
environment
with around 200 clients(not sure what you have). Their VTL vendor
suggested
a volume size of 20Gb. This eventually created ~15000 volumes. When the
TSM
server used the VTL, the overhead from mounting hoards of virtual
volumes
brought their server to its knees. I mean it would not even respond to
session requests so it could not complete nightly client backups at all.
Not to mention the headaches of managing 15000 volumes from the TSM
interface (GUI or CLI). They too were trying to backup directly to the
VTL
during nightly client backups. We had to return there management class
destinations back to a random access storage pool and process their data
during daily sever maintenance as TSM is designed to do. We set up the
VTL
to emulate LTO2 (200GB volume size) and used the VTL like a library,
migrating the nightly backup data to the VTL. Large Oracle backups do
directly to the VTL but are limited in number. The client is fat and
happy
now, backing up ~1.5TB nightly with plenty of time left and the TSM
server
performance is stellar.


Regards,

Nicholas

"ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU> wrote on 07/07/2009
11:18:26 AM:

> [image removed]
>
> Re: [ADSM-L] VTL Tape Size
>
> John D. Schneider
>
> to:
>
> ADSM-L
>
> 07/07/2009 11:19 AM
>
> Sent by:
>
> "ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU>
>
> Please respond to "ADSM: Dist Stor Manager"
>
> Andy,
> My experience may not map to the problem you are trying to solve, but
> I chose a relatively small VTL tape size (50GB) and have not regretted
> it. The trade-off is "total number of virtual tapes" vs "total number
> of anticipated simultaneous tape mounts".
> Say you have a 60TB VTL (usable), and you want to emulate LTO4 tapes.
> If you went with the default size (400GB) you would have about 150
> virtual tapes in your pool. Say also that there are 300 TSM clients to
> be backed up each night. Each one will need at least one virtual tape
> during their backups, and some of them might need 4 or 8 for performance
> reasons. You would have only 150 tapes for 300 clients? You could
> spread out their schedules, of course, but that will still be
> problematic. After a few weeks you might have a bunch of them full, but
> not ready to reclaim, or waiting on reusedelay, and not have enough
> available tapes for all the tape mounts you need.
> With 50GB tapes, you would have over 1200 virtual tapes. Tapes would
> fill up sooner, of course, but they could be reclaimed sooner, too, and
> be returned to scratch. Your overall disk utilization will go up.
> One thing to bear in mind is that if you have single files that are
> bigger than your virtual tape size, the file will have to span multiple
> virtual tapes. This is no problem for TSM, but it does mean that each
> of the virtual tapes involved in that one file will not be mountable
> until after that large file is finished backing up. We have seen the
> unusual situation where a single 300GB Exchange database was backing up,
> and happened to run over into our 'backup stgpool' window. The 'backup
> stgpool' was waiting on a tape mount of a certain volume, but when we
> checked we could see that the volume was not mounted or in use by
> anybody else. After some digging we noticed that the virtual volume in
> question had been mounted some hours earlier in a backup session for a
> single large Exchange file, and that backup was still going on. As soon
> as that file finished backing up, the virtual tapes mounted and the
> 'backup stgpool' continued.
>
> Another thing to think about is, have you sized the virtual library
> to have enough capacity for all your primary storage pool needs, or will
> the primary pool have to migrate to real tape? If so, that is another
> argument in favor of relatively small virtual tapes, because they won't
> migrate until they are full. In our case, using the migration
> threshhold to cause the migration to occur didn't work well because of
> how TSM calculates percent full, so we ended up writing a script that
> automatically migrates (using "move data") virtual tapes as they age, so
> that we are sure we always have enough scratch tapes for our next
> backups.
>
> Best Regards,
>
> John D. Schneider
> The Computer Coaching Community, LLC
> Office: (314) 635-5424
> Toll Free: (866) 796-9226
> Cell: (314) 750-8721
>
>
> -------- Original Message --------
> Subject: [ADSM-L] VTL Tape Size
> From: "Huebner,Andy,FORT WORTH,IT" <Andy.Huebner AT ALCONLABS DOT COM>
> Date: Tue, July 07, 2009 9:36 am
> To: ADSM-L AT VM.MARIST DOT EDU
>
> We are about to bring up new TSM servers and one of questions that has
> come up is how big to make the VTL tapes? We currently use 100GG and
> have tried 10GB with our test server.
> The question is what size it popular and why?
>
> Andy Huebner
>
>
>
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