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Re: [rdiff-backup-users] tape drives

From: Greg Freemyer
Subject: Re: [rdiff-backup-users] tape drives
Date: Mon, 23 May 2005 16:58:19 -0400

On 5/23/05, Susanne Hemker <address@hidden> wrote:
> Hi all,
> I am looking for a new tape drive for backups and couldn't find any
> good reviews online. Could anyone suggest a good drive for a regular
> backup of a ~800GB partition? Up to now we had DLT drive using 80GB
> tapes, but we  are looking for a large capacity drive. Since I have no
> experience with LTO drives any hints in that area would be really
> appreciated. Or any suggestions on smaller capacity drives using an
> autoloader.
> Thanks for your help
> Susanne

For anything that big, I assume you are primarily looking at SDLT,
LTO2, AIT (or SAIT).

I'm not as current with tape drives as I used to be, but I know a
couple of years ago, HP was recommending the LTO technology because of
its ability to slow down.

That can make your backup run a lot faster becuase it can reduce the
amount of shoe-shining that the tape drive does.

I don't know if any of the other drives technologies have added that
capability or not.

==== Why slowing down can help.

Assume you have a LTO-2 drive that can run at 30 MB/sec of compressed
data to tape speed.

Assuming 2x compression, you need to feed that drive 60 MB/sec of
uncompressed data.

HP recommended that you have a disk-subsystem that has the ability to
achieve 3x the tape speed to ensure you had enough data to stay in the
streaming mode of the tape, so now you need a 180 MB/sec disk

That is a lot of disk-subsystem speed.  (Note that 180 MB is not out
of the raw device, but out of the backup software which can be way
slower than the disk itself.)

So a simple disk speed test with "tar cf /dev/null ." can tell you how
many MB/sec your disk subsystem can produce.

Most systems have a hard time keeping up with the high-speed tape
drives available.

The LTO-1 drives from HP had the ability to slow down to as slow as
half speed, and I believe all manufacturer LTO-2 drives have the
ability to slow down.  This means that if your disk subsystem is just
a little too slow to keep the tape running at full speed, the tape
motor just slows down a little.  Other technologies used to kick into
the ultra-slow shoe-shining mode.

Greg Freemyer
The Norcross Group
Forensics for the 21st Century

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