Re: VTL - Fiber vs SAS

Well I do know there is SCSI overhead in there for sure but in theory the absolute maximum for LTO5 is only 280 MB per second uncompressed. This then is the amount of data that must traverse the link. That works out to only 2.35 Gb/s for the uncompressed data. That would put SCSI overhead on that at 121% if IBM's numbers are to be believed!! That's pretty rich.

Now I haven't analyzed this of course so it's potentially correct.

What you have configured of course is marvelously better and with LTO7 the theoretical bps is more like 6.3Gbps (raw and before SCSI overhead).

Moving to 16G and adding ports along with your SSDs should mean your backups are 'somewhat quicker' :-) :-)

As to the VTL the deal is it's THE SAME. The secret is going to be what can the VTL ingest before it queues up itself. A unit with RAID disk and only 4 drives (most of the minimum configurations) isn't going to be able to match an LTO7 more than likely maybe a single LTO6. Some of the more pricy units have SSDs in there and of course the bigger units have many more drives boosting ingest by a lot.

Nonetheless the IBM i side of the configuration is ALWAYS important and even with SSDs what you are backing up still matters.

- Larry "DrFranken" Bolhuis

www.Frankeni.com
www.iDevCloud.com - Personal Development IBM i timeshare service.
www.iInTheCloud.com - Commercial IBM i Cloud Hosting.

On 1/30/2020 2:25 PM, Steinmetz, Paul via MIDRANGE-L wrote:

Larry,

P9 9009-42A 100% SSD
Currently have an 3573 with 4 LTO7 HH FC drives connected from 2 LPARS via a Cisco 9148 FC switch.
Each LPAR has a EN0A PCIe3 16Gb 2-port Fibre Channel Adapter.
We're using both ports from the ENOA.
Each port uses 2 drives.
Switch is zoned so that each LPAR can see all 4 drives.

The SSD are most busy during a save.
The SSD response time goes from .1 to .8 during a save.
This was much worse with old spinny disks, (tape was waiting on the disks)

Back when we had LTO5 and from an old PMR,
An LTO5 drive will use 65% (per IBM support) of the 8 gb bandwith (5.2 gb), leaving only 35% (2.8 gb) for the 2nd drive.

It would be interesting to know how much bandwith a VTL would use and what determines the bandwith with a VTL.
Will a VTL use the full 16 gb bandwith?
Or does this depend on the VTL vendor?

Paul

-----Original Message-----
From: MIDRANGE-L <midrange-l-bounces@xxxxxxxxxxxxxxxxxx> On Behalf Of DrFranken
Sent: Thursday, January 30, 2020 8:44 AM
To: Midrange Systems Technical Discussion <midrange-l@xxxxxxxxxxxxxxxxxx>
Subject: Re: VTL - Fiber vs SAS

You are correct that 4 LTO6 drives all in all out max compressed throughput is roughly 13Gb per second. This is 2/3rs more than a single 8G port can support.

However it would be quite rare for most IBM i customers to have a sufficiently sized system to drive those drives to that level!!

That said, if you've invested in four drives you likely have a pretty big system and should do your homework on port speed, and for that matter the PCI-e Slot speed you're using!!

For THE VAST Majority of IBM i customers upgrading their tape drive beyond LTO6 is likely to result in zero reduction in backup time. Most wouldn't notice anything above LTO5. They just don't have the I/O infrastructure to push it.

Only reason I bring this up is I hate to see customers investing in LTO7 drives (And $50+ Tapes) along with 16Gb Fiber ports when a SAS card with
LTO5 drive (And $20 tapes) will do the backup in the same time!

- Larry "DrFranken" Bolhuis

www.Frankeni.com
www.iDevCloud.com - Personal Development IBM i timeshare service.
www.iInTheCloud.com - Commercial IBM i Cloud Hosting.

On 1/30/2020 8:10 AM, Holger Scherer wrote:

right. Then also not to forget to have the IBM i host have multiple NPIV capable ports to gain faster speed.
It's of not much use to have one 8GBit fiber port on the host and 4
fast LTO6 drives with one 8GBit (or faster) port for each drive. Been
there, done that ;-) -h

Am 30.01.2020 um 13:59 schrieb Rob Berendt <rob@xxxxxxxxx>:

One big difference between a VTL and one of those IBM 3xxx series physical libraries with multiple drives is that the VTL uses a single port on the NPIV capable switch even if it has multiple tape drives configured in it. The physical library has you run a separate cable from each drive in the library to the NPIV switch.

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