I've been struggling with a Quantum DLT7000 SCSI tape drive for the past
week, and I'm not sure where else to turn for help. The problem is
simple; the drive's hardware compression simply does not work when backing
up my system.
System particulars:
- 2x Pentium III CPU @ 500Mhz.
- 896MB memory
- RAID 5 array on IBM Serveraid 4H controller
- DLT7000 on Adaptec 2940W controller.
I need to backup several logical volumes on the raid controller. The raid
controller can easily support continuous reads at > 30MB/s and the tape
drive (a SCSI2 device) needs about 5MB/s to maintain streaming.
I have (3) of the tape units. All of them pass the full manufacturer
diagnostics, which presumably exercise compression.
I am using plain, old tar for backup (but have also tried dump/restore).
Regardless of archive application or block settings (and I've tried
dozens), the tape runs out of room at 34.9 GB. This is the uncompressed
capacity for a DLTIV tape. Compression is definitely engaged (the panel
LED on the drive is illuminated). Granted that the advertised 2:1
compression is optimistic, but I'd still expect to see _some_ compression
on a generic spread of data.
For a control, I turned off the hardware compression and ran tar with the
'z' flag to pipe through gzip. Everything fits with room to spare. A
back of the napkin calculation suggests that I'm seeing about 1.5:1
compression using this approach. The downside is that its about 3x slower
and creates an annoying amount of shoeshining. Software compression is
not fast enough to keep the drive streaming.
I'm about out of ideas. It stretches my credulity to believe that all
three drives (from very different sources) are defective. What on earth
can I do to get the hardware compression to show an advantage? Is it a
SCSI subsystem issue? Any suggestions would be greatly appreciated.
Steve
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