Hello, Carl.

At 4/12/99 01:23 PM -0500, Carl McNamee wrote:
>We have been using dlt4000 tape drives for a couple of years now and wanted
>to shorten our backup window so we decided to try the dlt7000.  When I
>finally received one and installed it on the system with all the latest DLT
>and Turbo Store patches we were surprised to find the backup times were
>exactly the same as on our DLT4000's!
>
>Puzzled by this I had several tests run; with compression, without
>compression, ect...... and each time the backup took almost exactly 30
>minutes.  The dlt7000 is connected to its own f/w scsi controller and is the
>only device in its card cage off of our 997/500.
>
>Has anyone on the list had a similar experience?  Any thoughts on why the
>tape drive would be so slow?

Because the tape drives are not the limiting factor in your backup speed.  It's
probably time spent fetching data from disk and preparation for sending to
tape.  Are you interleaving files on tape?  I know that some folks really don't
like that (Hi, Stan! ;) but the purpose for interleaving is that when multiple
files are being written to tape at the same time, the chances that those files
may be on multiple disk spindles goes up significantly, and thus the time to
read all the files goes down due to the more parallel nature of the disk I/O
that permits.  That's why RoadRunner by default uses an interleave of 4 and
allows the user to make that as high as 8 files concurrently open for storing
per tape drive.

Also, RoadRunner doesn't read a file which spans multiple drives in logical
sequence.  It is aware of which extents fall on which spindle and reads extents
on multiple drives concurrently as much as possible.  This also helps increase
the parallelism of disc reads during the backup.  I don't know whether
TurboStore also does that, but with the TurboStore default of no interleaving,
it's my understanding that the files are always stored in logical order on
tape...  hence the file has to be read one extent at a time in order.

If you use RoadRunner, which if I recall correctly you used to use there, try
increasing the interleave to 8.  If you use TurboStore, then add the ;INTER=
option and run your test again.  I would be very interested in seeing the
results.
--
Jeff Woods
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