From owner-freebsd-hackers@FreeBSD.ORG  Mon May 18 16:27:13 2015
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Subject: Re: Got seagate archive Was: Help with 8TB Seagate Archive
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>       slows down.
> 
> 
> 
> What sort of performance do you get when you use the drive as intended (i. e. tape-on-disk), either using tar or dd to raw device,
> i. e. where the writes are always linear and not scattered?
not tested purely linear writes, but writing large files result in 
slowdowns too. maybe smaller.

Ask someone to test speed with

dd if=/dev/zero of=/dev/seagate-archive bs=1m


and check speed AFTER 20-30GB got written.

in theory it should be fast.

in theory there is no difference between theory and practice. In practice 
it is ;)

Actually it should be OK if firmware is well written. In my opinion it is 
usable but not really well written.