ADSM-L

Re: [ADSM-L] Remote tape drives

2008-11-17 12:51:13
Subject: Re: [ADSM-L] Remote tape drives
From: "Allen S. Rout" <asr AT UFL DOT EDU>
To: ADSM-L AT VM.MARIST DOT EDU
Date: Mon, 17 Nov 2008 12:50:30 -0500
>> On Mon, 17 Nov 2008 11:09:55 -0500, Paul Zarnowski <psz1 AT CORNELL DOT EDU> 
>> said:


> We are just in the process of creating copy volumes which we intend
> to relocate to an off-site library in a few months.

I want to make certain I'm responding to this correctly; I've been
talking about remote virtual volumes.  You appear to be discussing
local volumes which you intend to physically transport, and then
contact via virtualized fiber.

If I've understood correctly, I think that your DR library is only
"remote" in the meat world.  You've got rmt devices on the local box
which terminate in physical tape drives "somewhere".  They're
different from the drives you deem "local" only in performance.  If I
get your aim, you are attempting to have TSM treat the 'remote'
volumes as though they are offline (in the Vault) and build new
replacement volumes.  You may be able to hornswoggle TSM into doing
this by manually setting State and Location on a bunch of volumes.


But I'm going to claim you don't want to do this, and the bandwidth
utilization is in fact the optimal solution.  The alternative,
building new volumes, is going to waste a -lot- of local tape time.
Each offsite reclamation process you go through, you're going to (to a
good appoximation) mount every single primary tape in your library.
Calculate mount delay * nvols as an absolute -floor- of wasted tape
hours per reclamation attempt.  And then your access is far from
optimal.  Lots of seek-read -- seek-read.  My offsite reclamation runs
persist for days and days.


As an alternative, -you- have the option of seeking along one physical
volume, feeding another one.  If you get the pipe right, you could
almost certainly maintain > 40M/s on even a single Gb channel.  It's
duplex, remember...


It would certainly be nice to have something SAN optimized, so you
could do a direct tape-to-tape copy with the whole data channel Over
There.  I wonder if that's in the works anywhere...

- Allen S. Rout

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