My info is old, but my guess it hasn't changed much since I used to work for
a VAR and we sold both TSM and Veritas NetBackup:
Veritas multiplexes blocks from various backup clients on the tapes when
this is being done. It works, and is very effective. They basically used a
hacked version of gzip to prepend the node name/sequesnce number
(effectively) on the data block before it is written to tape.
This is best used when writing directly to tape from multiple clients
backing up at the same time. If you keep the tape fed and you have enough
bandwidth, you can keep the tape streaming. For performance you MUST keep
the tapes streaming. (I think I heard that you may be able to backup to disk
now, and then backup to tape, but I never used it, and I may be
mis-remembering too.)
Scaling backups with Veritas has to do with bandwidth, tape streaming speed,
number of concurrent clients you are backing up, and number of tape drives
available to backup to.
TSM does this as a two step process. Backup to disk local on the TSM
machine, then put the files from disk to tape. This does not 'multiplex'
like Veritas does, but it intermingles files from various clients on the
same tapes. Since the disk drives are local, and if your TSM server is
configured correctly (I have one with a problem, so ...) you can keep the
tapes streaming by sucking the data from disk. (Before someone crys foul,
yes you can use TSM to backup directly to tape, especially for large files,
but I havn't heard of people using this much, and it is an issue if you need
to keep the tapes streaming and there isn't enough bandwidth).
This means that Veritas requires less disk and better bandwidth from clients
when backups are happening, and TSM can handle small clients, random backup
times, and low bandwith situations much better than Veritas. But TSM does
require the disk space, and a good database.
Veritas has the benefit of if the database 'catalog' is destroyed and you
cannot get the catalog back from a backup, you can re-bulid the catalog, but
you have to spin all the tapes, end to end. (Veritas seems to be an easier
'sell' because it is full+diff/incremental type of backup rather than TSMs
'incremental forever'. After 3years management here still doesn't have that
down!)
TSM and Veritas have the SAME problem when it comes to restores. They both
MAY have to spin a number of tapes to perform one (non-trivial) restore.
In general, TSM uses more disk, Veritas uses more tape drives, TSM must have
some 'downtime' for reclaimation. Database maintenance on both systems is
needed to keep things optimal.
I hope this helps ... Jack
-----Original Message-----
From: Gill, Geoffrey L. [mailto:GEOFFREY.L.GILL AT SAIC DOT COM]
Sent: Monday, July 19, 2004 11:06 AM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Multiplexing to a single tape
I was wondering if anyone was multiplexing backups to a single tape.
Recently I attended a Veritas class, our "other" new backup system, (yes I
did ask to stay completely with TSM) and they really touted this as what
seemed like a very good means of backup. Unless you have the newer faster
technology tape drives I'm not sure I agree with it but don't have
experience since disk pools have always been plentiful. I'm not sure I agree
with it anyway because I believe restores would be slow since data would be
scattered throughout the tape. What if multiple restores need the same tape?
Can they run at the same time and if so wouldn't that be even worse?
So the question I have is, has anyone tried this functionality with TSM, or
Veritas for that matter since I believe there are others on the list that
use it. I am interested in any feedback so feel free to praise or rail on
it.
Thanks,
Geoff Gill
TSM Administrator
NT Systems Support Engineer
SAIC
E-Mail: gillg AT saic DOT com
Phone: (858) 826-4062
Pager: (877) 854-0975
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