In article , Boll Weevil ruminated:
Yes, you can share the HBA with disk and tape but it is not recommended.
Disk does random read/rights while tape does streaming reads/writes. If
your server gets high I/O like in a production invironment, your backups
will eventually have problems. The disk traffic will interrupt the
backup traffic and will either cause shoe-shining and eventual backup
failures.

Modern tape drives have fairly sizable buffers. The bigger problem will be
whether you exceed the bandwidth limits of the bus, in which case your
scenario plays out. But if the HBA has enough bandwidth to handle both the
disk storage and the tape drive (i.e., typical small server environment),
it's not an issue. When writing DDS4 tapes (which are quite slow, app. 3mb/sec
max bus bandwidth), I never ran into problems with disk contention (granted,
disk was a single 7800rpm Barracuda 18gb hard drive, tells you how many
years ago that was, eh?). Granted, this was with Linux, which used a simple
FIFO for issuing SCSI commands... in fact, on Linux, tape I/O was more likely
to cause contention problems than disk I/O, because tape I/O was typically
done in bigger chunks (32K or more).

A bigger problem is if your tape drive doesn't have a high speed SCSI
interface, in which case it'll slow down the entire bus to the speed of
the tape drive's bus.

This is not to say that a fully packed server with a high speed SCSI RAID
can be set up like this. Obviously you *will* end up with contention
problems in that case, because a high speed SCSI RAID subsystem can swamp
the SCSI bus under extreme load. But it's all a matter of bandwidth and
OS design. I'm sure that if you design your OS really shoddily (like some
so-called "professionally designed" commercial OS's) you can make it so that
tape doesn't work right when you have it on the same bus as disk. But (shrug)
I don't spend much time worrying about defective operating systems (I use
Linux and FreeBSD).

Also, if you ever want to do server-less backups, you must
have a second HBA.

Why? Please explain.

--
Eric Lee Green
Linux/Unix Software Engineer seeks employment
see http://badtux.org for resume


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