Flexraid is closest to raid 4
Every file is completely written to one disk, not spanned across multiple disks.
This gives you the best read performance (maximum read of existing hard drive, no software raid calculation to logically rebuild each block before reading)
If you used this with real time raid, you'd have pretty poor write performance since everything you write is redundantly written to a single parity drive.
To make the best of this, it's often suggested in these forums to use a snapshot mode where the parity rebuilds on a schedule and any new data added between rebuilds is "at risk" of being lost in the even of that disk failing.
This is one of the tradeoffs you decide when choosing realtime or snapshot, and I'd take the improved write performance and stability of snapshot mode every time. Most of my data is already written, so any new data written within 24 hours shouldn't be that hard to recover.http://www.thegeekstuff.com/2011/11/raid2-raid3-raid4-raid6/