[Veritas-bu] Long distance fibrechannel tape drives

I've not tried this, but it is possibly a shortage of buffer credits on 
the metro link.  I'm also not a CISCO user, but assuming that all SAN 
switches are the same :-) you will have configured that particular port 
for long-distance working.  That will allocate more buffer credits than 
normal, but maybe not what you need for sustained tape traffic.

I assume you have switches at both ends (i.e. not just the tape library on 
the remote site).

You need in principle one buffer per km distance to keep the fibre full of 
data.   I'd suggest seeing if the switch has the ability to measure the 
actual link, rather than just the as-the-crow-flies distance.  The switch 
should have a way to control and view the credits on each port.

The link is dedicated to this purpose?   Otherwise you may have a 
bottleneck anyhow.

William D L Brown




"Bahnmiller, Bryan" <BBahnmiller AT pier1 DOT com> 
Sent by: veritas-bu-admin AT mailman.eng.auburn DOT edu
24-Oct-2005 23:35
 
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Subject
[Veritas-bu] Long distance fibrechannel tape drives






All,
 
  Has anyone else attempted to run tape drives over a long distance 
FibreChannel connection?  We are trying to run backups to tape over a 
metro FC connection. It works â?? slowly. I was wondering, before we spent a 
lot of time down at a very low level â?? if anyone else has tried this 
successfully before.
 
  We are running NBU 5.1mp3a on a W2K3 master with AIX media servers. We 
are running to SAN connected tape drives locally and remotely. The tape 
drives are IBM LTO2 in ADIC Scalar I2Kâ??s. The SAN is Cisco MDS9000 series 
switches. The remote data center is connected to the local one via long 
distance FC (approx 23 miles.) 
 
  We have set up a local media server to run tape backup jobs to the local 
tape drives as well as the remote drives. With a standard setup of 
clients, server, and policy we will see upwards of 50 MB/s going to a 
local tape drive. With the exact same configuration, except changing the 
storage unit to the STU with the remote tape drives, we will see about 20 
MB/s to the remote tape drive.
 
  Is this due to the latency in the long distance over Fibre Channel? Or 
are we missing something?
 
      Bryan
 
Bryan Bahnmiller
ISD Business Continuity
Pier 1 Imports, Inc.
Pier 1 Place
Fort Worth, TX
817-252-8570