after I tried out actually every omv-supported file systems able to handle more than one HDDs in a single volume, from my humble opinion there remain only two file systems, that are reasonable so far: ZFS, the non-Linux mother and btrfs, her real Linux child.
I will use btrfs in future, as it supports an implicit RAID5 or RAID6 like ZFS. And even if already hear the cry: "But the write hole...." Sorry, but the write hole is an issue not only of btrfs, but of ZFS as well, and in fact of *every* RAID (http://www.raid-recovery-guide.com/raid5-write-ole.aspx) Think it's not really difficult to understand: *Any* redundant saving needs at least two writes to disk to be fulfilled. Which must occure at exatcly at the same time at the platters involved, if a sudden power outage shoudl do no harm. But which OS existing can guarantee this? And even if the OS would do it - how can we be sure, that our disks write their buffered date at the same time? Or at least in the time, they still have, to do anything, if power suddenly fails?
So there is a write hole not only by theory, but by hardware. And there is only one solution: a UPS able to let the drives write all data they got before shut down. With today's drives buffering 100MB or more we even cannot rely on BBUs on drive controllers no more. They only can save data not sent to the drives yet. But they cannot recover data, that are sent to the drive's buffers already.
So what at all - to be safe, for every setup with >1 drives driven parallely we need uninterrupted power, as long as the writes last. And a UPS for the whole system is the only solution against write holes. Regardless, which file system and drive pool organisation we use.
Btw: Btrfs is used quite normally for RAID5 and 6 setups by Synology and Rockstor. Both recommend to use a UPS. Of course.
Btw2: I actually don't beleive, that the write hole ever can be closed. Neither in btrfs, nor in any other file system. Just think, this would need multiple disc writes and confirmation at the same time. I don't beleive this possible with a power fail inbetween.
Thanks for reading my thoughts and