Format your DSSUs with a 64k block size.
Use 64K block size on raid group.
Disable compression (if you can't use 64k block size.)
Lower the number of concurrent writes to the DSSU.
Occasionally "clear" the DSSU to relieve fragmentation.

Clear the DSSU, write a single backup to the disk, then stream the backup to 
tape. Whatever performance you get from that single stream is the best you are 
going to get from that configuration.  Any additional concurrent writes will 
likely slow your performance.

We had a lively discussion on this topic a few weeks back. Here's a link to a 
post I made about fragmentation and wet blankets.

http://www.adsm.org/lists/html/Veritas-bu/2010-05/msg00029.html

-Jonathan

-----Original Message-----
From: veritas-bu-bounces AT mailman.eng.auburn DOT edu 
[mailto:veritas-bu-bounces AT mailman.eng.auburn DOT edu] On Behalf Of 
WALLEBROEK Bart
Sent: Wednesday, June 23, 2010 9:23 AM
To: veritas-bu AT mailman.eng.auburn DOT edu
Subject: [Veritas-bu] Destaging going slow

We have a couple of windows (dedicated) Media Servers that act as NetBackup 
DSSU (Disk Staging Storage Unit) servers with each having from 4 to 16 TB of 
SAN disks.  Backing up to them is no issue and goes up to the limit of the 
network connected clients.  Destaging to tape (LTO4) however is slow (20 - 35 
MB/sec).

We have been in contact with Symantec where they advised us to lower the 
fragment size, change the data buffers size and numbers but we seem to get 
stuck to the current speed.  Backing up data from this DSSU disk to the tape 
drives goes up to physical tape speed limitation.

Does anyone have an idea where to look further to get to a reasonable speed ?

We currently are testing a Solaris DSSU server but we do not have numbers for 
that one.

Best Regards,
Bart WALLEBROEK
Backup Admin & Systems & Applications Management & Support Specialist 
Enterprise Applications Delivery - Infrastructure Management
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