I agree with all the other responses as to the various reasons that your
drives aren't getting used.  I'd like to take a different tack with my
response.

 

I'd like to suggest that using all your tape drives is often not a good
thing.  I would suggest that instead you should worry about how many of
them you are using EFFECTIVELY. Let me give an example using LTO-2
drives, GbE, and a regular master/media server doing backups across the
LAN.

 

If you've got 50/60 MB/s coming into the GbE interface, and nothing
more, and you can fill that pipe with one LTO-2 tape drive (40 MB/s *
1.5:1 compression = 60 MB/s), then why do you need to use more than one
tape drive?  (Unless you're using Inline Tape Copy.)

 

If you take 50/60 MB/s and divide it by 5 and give that to 5 LTO-2
drives, 50-60 quickly turns into 25-30.  Why?  Because 50/60 divided by
5 is 10-12 MB/s, and an LTO-2 tape drive can't write at 10-12 MB/s.  It
wants to write at 50/60 MB/s.  If you send it 10, it will spend all its
time shoeshining and very little time writing data.  10-12 very quickly
becomes 5-6 MB/s due to all that shoeshining.  Then multiple 5-7 times 5
drives, and you've got 25-30 MB/s.

 

You have 50/60 available, and now you've got 20/30 actually happening.
AND you're wearing out your tape drives, media, etc, with all that
shoeshining.

 

IMHO, the best thing you can do to ensure successful backups and
restores (short of moving to VTL) is to match the speed of your tape
with the speed of your pipe.  If your incoming network can only support
one tape drive, then that's all you should be using.  If it can stream
two, then have a blast with two.  But if it can only stream one or two,
and you're using 5-10 drives simultaneously, you're just banging your
head against the wall.

 

---

W. Curtis Preston, Author of O'Reilly's Backup & Recovery and Using SANs
and NAS

VP Data Protection

GlassHouse Technologies

 

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