Hello @Goran K ,
most tape libraries are FC based, but there are some SAS connected available.

What amount of data do you expect to be written to tape. You say you have 500TB source data. The amount of backup data depends on your retention time, RTO and so on.

Several vendors offer tape libraries that fulfill your requests, e.g. IBM, Quantum and others.

@JMeixner Thanks for reply, i am of course aware of these parameters and i know from experience that we can ditch TS3500 so i was wondering what others are using in what setup so i can start contemplating 

I think you are going to find a variety of setups out there and all being dependent on budget constraints, etc.  Also if you want all that data on tape you would want to look at LTO9.  This link is great for reviewing speeds and capacity which I use -

https://www.ibm.com/docs/en/ts4500-tape-library?topic=performance-lto-specifications

I agree with LTO9 but 500TB?

Are you sure only 2 tape drives is going to be enough? Whats RTO/RPO.. It’s going to take some time to get that all on tape and if you need it back concurrency is where tape can really excel. 

I wish I could still find an ISCSI-based tape array….apparently they used to exist based on my previous research last year, but I couldn’t find any currently. 

our choices are either Fiber Channel, external SAS (I believe) or internal (via SAS).  In my case, I had a small client needing a single LTO7 tape capacity, so I put an LTO8 drive (for future proofing) into a small tower server.  In many cases, it wouldn’t surprise me if you could easily put a couple of drives into some of the servers, but the internal drives do require 5.25” drives and I only needed one.  I’m not a HPE guy, but since the client is a HPE shop, I did have to work with our distributor to assemble a server that would act as the tape server.

As noted, for larger deployments, there are very large tape libraries (I know you said this is a smaller solution, but it’s much larger that what I needed), but as Joe noted, Quantum, IBM and such (is Qualstar still around?) do make tape libraries and some of them are as small is a single to 2 or 4 drives with bays to hold around a dozen or so tapes.

I didn't want to do advertizing a special product here…

The TS3500 goes End-of-Support at the end of the year, so it is no good option. And I am afraid a TS4500 is a little bit oversized.

An alternative for your size of environment could be a TS4300. This consists of a base module and up to six expansion modules. It supports LTO-9 drives in the meanwhile…

But to be clear, I am no IBM representative and I don't want to advertize. These models are just the ones I have the most experience with…

They are a leader in the tape libraries though, however there are other large players in the space. I’m not an IBM rep either, but I did used to work there for full disclosure. With their last round of layoffs many of my old friends got let go and I have no loyalty to them however.

I run 2 TS4500’s here, and they have been incredibly solid. We came from a 3500 which was also a very solid machine. We did have issues with an older TS4500 but they were minimal. If I was to get 2 more I’d get TS4500’s again. I have the drives full all day and they run 24/7. 

I have no experience with the 4300 but if you don’t need something huge it could be an option as @JMeixner has stated.