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Slow backup with superloader3

Level 3
Not sure if this is the right board. Please move posting to the right location, if not.

Our system (Dell server):
Xeon 2.8GHz
1 GB Ram
Perc 4 Raid controller
Raid 5 with 8 x 146 GB 10.000rpm drives
Adaptec 39320A SCSI controller with only the tape drive conected on external channel A
Quantum Superloader 3 (LTO-3), using the symantec driver (Version 5.1 is displayed)
Windows 2003 SP2 with all MS updates 'til April 2008
Backup Exec 11d 7170

The Server is ONLY for backup, so no other services or programs are running on it. CPU usage during Backup is 60% max. in peaks only, around 30% most of the time.

Data backed up: 550GB in ~1million files. Mostly MS Office documents (doc, xls, ppt), pictures, and a few database dumps.

The problem:
Data transfer rate is low. We are only getting ~800 MB/Min as an average over the whole backup. The backup starts with ~2000MB/Min and is then continously getting slower. Until last week we had a SDLT-320 running on this server, getting ~750MB/Min. So this wasn't really a big improvement. As we have another backup server on a co-location with the same hardware but only a single Quantum LTO-3 drive we expected to get a transfer rate around the 2500MB/Min we are getting there. The data backed up on the co-location is nearly the same (Office files, database dumps, some CAD projects) and ~700GB in ~1million files. Drivers on both systems are the same version.

What we tried:
Different settings for buffer size, buffer count, high water mark, even running in single block mode. No significant change in transfer rate.
Other drivers for the Adaptec controller (MS integrated vs. actual driver from Adaptec).

Maybe anyone can give me a hint where to look, or what to change to get this running a bit faster. Any transfer rate around 1500MB/Min would be great. Maybe there is an "approved" setting for the Superloader 3? Maybe it would help upgrading to Backup Exec 12?


Edit: forgot to mention: hardware compression is enabled. Old drive showed 1:1,3 on backups. New drive shows 1:1, BUT tape (400GB) has 20 GB free after backing up 550GB, so, if I am not completly wrong, this would be around 1:1,44 ratio.

Message Edited by Martin Baertl on 05-23-2008 01:13 AM