Thanks to everyone who responded to this thread. It was the simple solution:
storage unit definition for max concurrent drives. We are using inline tape
copying, and our remote storage unit was limiting the amount of tape drives
used.
Curtis' response is particularly informative, and I suspect that he is
correct that the amount of tape drives used may not be the bottleneck.
On 12/8/06, Curtis Preston wrote:
>
> 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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