[Veritas-bu] Long distance fibrechannel tape drives

William, all,

  Thanks for the suggestions and comments everyone. 

  We do have switches at both ends of the link. Buffer credits were set
to "auto" with maximum available. The distance on the link was reported
by word of mouth to me as "about 23 miles" but I have not seen an actual
measurement. We can do a FC ping, and source to destination is about
1000 ns. We are going to set the buffer credits at both ends to a fixed
value and see if that makes any difference.

  We connect to the link vendor's hardware in our building - they bring
in the fiber and the equipment, and we connect to the port with a 2 Gb
shortwave connector. I'm assuming that they are doing some sort of
conversion behind the scenes from us - they obviously have to do
something in order to cover the distance, I think it is DWDM. 

  Currently our link is a dedicated link. We may use it for other things
down the road, SRDF or such, but currently we are testing our options.

  By no means do I fully understand the interaction of how the NetBackup
data buffers eventually get broken down to the FC data packets. Or how
the handshaking occurs at that level. My assumption is that has
something to do with the lower data rates over the longer distance. 

  So far, we have only been able to push the 2 Gb fiber to about 10% of
its capacity with NetBackup going to tape drives. So we still have a lot
of empty space on the fiber while our packets are in transit. But we
definitely have more than one packet on the pipe at a time, otherwise
the speed would be much worse.

  I know the speed of light is still an annoyance to a lot of us, but we
try to work within the limits...

   Thanks all, I'll keep digging.

       Bryan

-----Original Message-----
From: veritas-bu-admin AT mailman.eng.auburn DOT edu
[mailto:veritas-bu-admin AT mailman.eng.auburn DOT edu] On Behalf Of
william.d.brown AT gsk DOT com
Sent: Tuesday, October 25, 2005 1:52 AM
To: veritas-bu AT mailman.eng.auburn DOT edu
Subject: Re: [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
 
To
veritas-bu AT mailman.eng.auburn DOT edu
cc

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 - slowly. I was wondering, before we spent
a 
lot of time down at a very low level - 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 I2K'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
 


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