(My origional post was rejected . . . I'm resending it)
I've been playing around with the same thing although in a completely
different context.
Although a better person should answer this, I believe the occ is what the
TSM server sees -
the byte stream into and out of the TSM server. If running client
compression, then the
server only sees the compressed data stream coming in. If compression at
the tape
drive, then tsm only see the data stream it sends to the drive, not what
the drive does.
But . . . this gets more complicated.
Remember that the CDL emulates tapes - in all their details. This includes
the need
for expiration and reclamation. In other words, how full are your tapes?
If you are running
reclamation at, say, 60%, then you have up to 40% of many tapes unusable
until reclamation
is run. The CDL will work exactly the same way on it's virtual tape
volumes. My
understanding is that when a new virtual tape is created It grows the disk
space allocated
in chuncks (I thought I heard 5gb as a number once), so new tapes only use
used space.
But full tapes with expired data still use the full tape.
So, your calculation might have to be something like
. . . . .(thinking out loud . . . high possibility of error) . . .
( total-occupancy + total-expired-but-not-reclaimed-space ) /
virtual-tape-drive-compression-ratio + some huge fudge factor.
It is very possible with the CDL with growing new tapes by chunks to
overcommit
the CDL. You could end up with a situation with many TSM tapes that are
FILLING,
but have the CDL out of disk space. It does have the option to fully
allocate the disk space for a virtual tape when the tape is created.
Another thing to think about . . . . co-location. I've been thinking about
this a lot. If
we used s CDL, would I still want to co-locate our primary tape pools?
Virtual tapes
should mount very quickly. Seeking to the data on a virtual tape should be
fast,
although I haven't heard how the CDL implements seeks. So, can I do
without
co-location on the primary tape pool? I don't know . . .needs more
thinking and
probably testing (if we ever do this).
Another thing to think about . . . . which library to emulate and how
many. The list
of libraries you can pick from to emulate is small. Since a storage pool
has to
live within one library, the number of virtual tapes in the library has to
have the
capacity for the storage pool (again, don't forget expired data). The CDL
has a
limit on the number of virtual volumes, libraries and drives. According to
the data
sheet from EMC's web site, the CDL supports "Configures up to 32 tape
libraries,
256 tape drives, and 2048 cartridges with a single disk library system . .
". You are
going to have to juggle your pool sizes, the library size and the number of
virtual tapes.
Much to think about. . . . . . .
Rick
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