>I'm looking to know what I could *realistically* expect from a 3590e drive
>during a *restore*, assuming the tape drive is the bottleneck, ie. disk is
>fast enough, bus is wide enough, everything is local attached, machine is
>super zippy, and restoring BIG files.
>
>Certainly the theoretical max is something along the lines of 14 MB/s, but
>I'm wondering how close people really come to that?

Oh, no! The Restore Question again!  Check the list archives for many
discussions on this topic.
There is no simple answer because the circumstances of the restoral can
vary so wildly.  Backup/Restore product vendors typically use large-glob
examples (e.g., huge databases) to be able to promote their product
based upon performance with one or a few very large files.  That's
obviously deceiving in that it's just a big streaming operations with
almost no overhead.  In the real world we have thousands or millions of
files of all sizes, spread over a considerable period of time and tapes.

Establishing a file in a file system takes considerably more time than
simply reading an established one, and so you can expect restorals to
take considerably longer than backups of the same data, all other things
being equal.

Tape is a bottleneck as compared to disk when restoring small files
spread over the tape, given tape repositioning time.  But in backing up
or restoring a single large file, disk is the bottleneck because of
its rotational positioning delay, as compared to tape, which is always
at the right position for the next byte of the file.

In the ADSM/TSM standard paradigm of incremental forever, you should not
expect to approach anything near maximum tape throughput specs in
restorals because of all the other things going on: tape positioning to
next file, TSM db lookups, operating system process dispatching in a
mixed environment, file system overhead, etc.  I don't believe that any
experienced customers even consider tape throughput specs in daily
practice because to many reality factors contribute to make it impossible
to attain the maximums.

With TSM you have myriad choices about how to set up your arrangement for
optimal performance in one way or another, based upon how much you want
to spend.  The flexibility is the beauty of the product.  You can go all
the way from basic incremental with no collocation so as to be cheap with
tapes at the expense of restoral performance, all the way up to volume
images for optimum restoration speed.  It's all a matter of your business
case choices.

   Richard Sims, BU

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