Impact of LTO 5 or 6 connected on 4 gb FC port

[Gurjeet_Singh]

Hi Team,

We are installing new i500 library, where we have LTO6 FC tape drives. these drives supports 8 gb fc, but we don't have 8 gb fc ports on SAN,

if we connect them on 4 gb FC ports, what could be the impact ?

Tape drives :- HP LTO-6 Tape Drive Module, 8Gb native Fibre Channel

speed :- 160mbps

capacity:- 2.5 TB

Master server :- NBU 7.5.0.4 running Linux 6.2

Regards

Gurjeet

[Yasuhisa_Ishika]

4Gb link is enough for single tape drive. But you should think about this based on your SAN topology. 4Gb link is not enough to connect multiple tape drives to a media server with single FC link. If you need to connect multiple tape drives to your media server, you should upgrade your SAN to 8Gb, or reconfigure SAN and data flow so as not to fill up bandwidth.

[Gurjeet_Singh]

Yasuhisa,

If I could understand you correct, as one drive max writes  160mbps, so 4 gb connections is more then enough, 

now media server is connected on 8 gbps FC, & drive is on 4 gbps, if I connect 5 drives to one media server than 160*5 = 800 mbps where media server is on 8 gbps server connection can give 125*8= 1000 mbps 

We are good, right.

although tape drive has only one leg which eighter can go in Fabric A or B, 

Regards

Gurjeet

[leonard_boyle]

using the ibm url

http://www-03.ibm.com/systems/storage/tape/oem/lto6/full-high/specifications.html

sending compressed data to the tape drive it does the following, note this is bytes not bits.

Native data transfer rate	160 MBps; 576 GB/hour

if you get x2 compression, that rate will double.

for easy math is you say 10 bits per byte then that is 1600mega bits per second or 3200 for 2x.

so the 2gig bits sec will work for one tape drive maybe two if you are like most of us and have systems that have a hard time driving the newer tape drives.

[Yasuhisa_Ishika]

8Gbps link can tranfer up to 800M bytes/sec because of 8b/10b encoding.

As leonard pointed out, tape drives with hardware compression sometimes feed more data than shown as native speed in spec sheet. But it depends on data characteristics, and what is more, if you can feed enough data throughout data flow from client to tape drives.

Estimstion based on native speed is good starting point. But if you are greedy for performace, it is better to use multiple FC link from the host and SAN switch(s), and zone one or two tape drives for each host link.

[Gurjeet_Singh]

Yasu leonard,

There are two aspect,

1) Media server to switch

2) Switch to tape drives.

here in my environment we have most of the FC ports running 4 gbps. if we upgrade some ports to 8 gbps &

we can connect media server to 8 gbps fc port  (800M bytes/sec)

& connect tape drives on 4 gbps fc port. LTO6 (160M bytes/sec) & IBM TS1140 (250 M bytes/sec)

if we connect 5*LTO6 to one media server (160*5= 800 M bytes/sec)

or

we connect 3*IBM TS1140 to one media server (250*3=750 M bytes/sec)

what is your openion will we be able to take max perfomance of the drives ?

& I believe if we use compression it will increase capacity & reduce throughput. Correct me if I am wrong.

Regards

Gurjeet

[teiva-boy]

You'd be hard pressed to get over 100MB/s to a single drive without some level of multi-plexing.  The minimum speed on those drives is around 40MB, yes it can go faster natively and even faster still with compression factored in.  

The point is that 4GB FC is usually not your limiting factor.  It's usually media servers, ethernet bottlenecks, and client bottlenecks.  Faster tape is such a case of diminishing returns...

[Gurjeet_Singh]

Team, FYI, I asked the same question to IBM as well

Question :- IBM TS3500 hasTS1140 IBM enterprise drive has capability to connect to 8 gbps FC ports. Could you help me to understand if we connect them on 8gbps fc port what the benefit we are getting?

& if we connect it on 4gbps port what are the pros & cons.
 
What is IBM recommendation on it?

Answer :- 

I would say the  difference difference  8Gbps vs. 4Gbps is bandwidth, and thus potential performance benefits.  If 8Gbps connections are easy to implement I would recommend using them.

With 8Gbps connection you get double the bandwidth to tape drive compared to 4Gbps.
The native transfer rate of one IBM TS1140 tape drive is 250 MB/sec, so with non-compressable data  a 4Gbps connection between a tape drive and server  would be enough (tape drive read/write speed would be the limiting factor here).  

But, if the data written to tape drive is compressible, we can then read/write actual data at much higher rate  to tape. TS1140 tape drive compresses data automatically  if it detects that data can be compressed
       -With 1,5:1 compression ratio  data is read/written to tape at 375MB/sec
       -With 2:1 compression ratio data is read/written to tape at 500MB/sec
       -With 3:1 compression ratio data is read/written to tape at 750 MB/sec

So when reaching compression ratios 1,5 or more you will benefit from 8Gbps connections.
A single TS1140 tape drive is capable of reaching almost 700MB/sec transfer rates.


One benefit for using 4Gbps for some customer is cable length; with 8Gbps the allowed cable length is shorter:
       -With OM2 cable and 8Gbps you can put 50m cable between two Fibre Channel devices while, while with 4Gbps you can put 150m cable                         between two Fibre Channel devices
       -With OM3 cable and 8Gbps you can put 150m cable between two Fibre Channel devices while, while with 4Gbps you can put 380m cable                 between two Fibre Channel devices

Question :- If cable length is not criteria, then 700mb/s is also achievable on 4 gbps port, even in practical it’s not achievable
These drives are connected in both SAN, One 4gbps cable will go to SAN1 & another 4gbps to SAN2. I hope these are load balancing mode.
 
Do you still see any performance issue? Do you seen practical scenario where these drives are utilized more than 700 mb or so…

Answer:

Only one tape drive port is active at one time. To  reach more than 350-400MB/sec you need 8Gbps connection.
Control path failover, data path failover and load balancing  are possible to use, they can be enabled by purchasing Path Failover license to TS3500 library/libraries . There are operating system restrictions on how those features can be used.

Reaching high data rates  on a single tape drive depends very much on what kind of data that is being backed up and the backup application itself (compression/streaming/multiplexing/block size used with tape media etc...).  Using more tape drives also typically improve performance. I have seen customers doing  500-600MB/sec on single tape drive, then again I have seen customers who can only partially utilize the high performance of TS1140 tape drives.