ADSM-L

Re: [ADSM-L] VTL Tape Size

2009-07-07 12:19:21
Subject: Re: [ADSM-L] VTL Tape Size
From: "John D. Schneider" <john.schneider AT COMPUTERCOACHINGCOMMUNITY DOT COM>
To: ADSM-L AT VM.MARIST DOT EDU
Date: Tue, 7 Jul 2009 09:18:26 -0700
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



This e-mail (including any attachments) is confidential and may be
legally privileged. If you are not an intended recipient or an
authorized representative of an intended recipient, you are prohibited
from using, copying or distributing the information in this e-mail or
its attachments. If you have received this e-mail in error, please
notify the sender immediately by return e-mail and delete all copies of
this message and any attachments.
Thank you.

<Prev in Thread] Current Thread [Next in Thread>