If you are driving the spinning disks that hard, then you aren’t using the right infrastructure.
With cache, automatic load balancing (by separating high I/O tasks on different groups of spindles), and using RAID-1 or -10 where you actually need performance, RAID-5 and RAID-6 disks groups should never see that kind of workload over the long term. In addition, you can mitigate almost all of the RAID-5/6 read then write problem with the right stripe size and an OS that is aware of the underlying storage. An OS will then hold data in cache until it has either a full stripe to write, or the disks are idle anyway, thus making the write penalty meaningless. The on-board cache of the RAID controller can also do this, even if the OS is unaware of the underlying storage.
This has definitely gone out of hand… This topic was about advice for an NAS used with the Vero 4K. Could the thread get back to the OP’s topic, please? 
Or has your question already been answered sufficiently, @ditlev?
I see folks recommending different options without saying any prices. I can’t speak for the OP, but price is usually THE primary concern for my home solutions.
Perhaps I missed the OPs requirements for a NAS too?
- How much usable storage? 20T of raw could mean 20T usable or 10T or less if RAID1 AND backup storage has to be included.
- Are backups included or a different problem solved elsewhere?
- Is the storage primarily for media? Media storage doesn’t need to be fast. 4K video should stream fine from USB2 storage.
- Does the storage need to perform other duties besides just networked storage? Plex Transcoding, perhaps? Thousands of network connections? VPN server?
- Which protocols are required vs nice-to-have? NFS, CIFS, FTP, sftp, rsync, iSCSI?
- How much skill does the OP have? Someone with 20 yrs of Unix admin is likely to choose differently than someone visiting Best Buy for a NAS. Everything takes longer than expected to accomplish.
An old WD TV Live HD with a single external USB2 disk could be called a “NAS” for $30-ish dollars. Performance isn’t great. 100base-tx isn’t great. Protocol support isn’t great.
Spend $800 (box only, no storage) on some video-editing NAS and you get much more.
Or did I miss this information sprinkled above?
BTW, I’m a CIFS hater for connections to OSMC. NFS over a wired connection just works, assuming we aren’t streaming content from the internet already. But everyone has their own requirements.
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I’m a CIFS hater as a developer of OSMC
Avoid when possible.
Whatever has to do with Software where Microsoft or Apple has its fingers in - avoid if you can.
These companies want to make money with their software, hence won’t open it as expected, and when they perform changes - problems are most of the time part of the experience.
Since I stuck to Opensource type protocols and technologies, my experience is that problems (if they show up), only tend to show up when you upgrade the system major release and some things have changed under the hood. For example going from nfs v3 to nfs v4. But very honestly, there usually is a fallback/compatiblity mode/flag to be set and it works again.
For network Filesystems: Go with FTP (I know, not secured - but for a LAN enough).
Also note that options like compression and encryption cost lots of CPU Cycles.
For example, I use rsync back in time to sync my drives using options without thinking …
Average transfer speed 30MBytes/s… I then checked the details why it wouldn’t go fast and realized that I had compression active (This was a local disk to disk copy, actually my montly backup on an external Raid 1 connectedm through E-SATA). After removing the compression, I all of a sudden had up to 140MBytes/s.
Lesson learned: that with a 1Gbps network, don’t activate compression. It will be faster 
on that note - any way to use iscsi with Vero?
OSMC is Debian based so the sky is the limit 
iSCSI doesn’t buy you much directly from the Vero…it would just make the remote disk shared by the iSCSI server look like a local disk on the Vero. The problem with that is you need a special shared filesystem if you want more than one device to mount the remote disk.
An NFS mount would give you about the same performance, and is designed to be accessed by multiple machines at the same time.
But, if you have an iSCSI target you want to mount, here’s how to do it in Debian:
https://wiki.debian.org/SAN/iSCSI/open-iscsi
Hrmm, never had a problem with SMB shares. Hell I have a drive on my windows machine auto mounted via fstab on my mythtv backend/fe. I then reshare that out via SMB so it appears as it’s coming from the mythtv be/fe.