iscsi параллельный доступ / iscsi совместный доступ объема на NAS
Concurrent access to a block device like iSCSI is not possible with standard file systems. You'll need a shared disk filesystem (like GFS or OCSFS) to allow this, but in most cases the easiest solution would be to just use a network share (via SMB/CIFS or NFS) if this is sufficient for your application.
Edit: This is getting too long for a comment.
You'll end up with a multitude of problems and I guess you won't be able to get this to work with what you got.
Your first problem will be the RAID itself. I don't see that you get a sustained write rate of >250 MB/s with three separate data streams on a simple 7 disk NAS RAID5. This might work for one data stream, but not for multiple concurrent streams.
The network. I don't know how many interfaces the NAS box has, but you'll get around 100MB/s of data through a single 1GB/s connection in the best of cases so unless you have at least three interfaces, the network will already limit you. And even if you have that many interfaces, the NAS has to allow to map them all to the same iSCSI target or allow to trunk them into one (which in turn require support at least from the switch).
If this is all somehow working, you can pick some shared disk filesystem that supports Windows and try to implement a cluster filesystem with this, but this FS will have some overhead of it's own, further limiting you.
Honestly, you should have got a consultant design a solution for your requirement before buying stuff, this is far beyond the simple office file sharing most NAS devices are designed and optimized to do.