Synology Hybrid RAID (SHR) how safe is it really with 2 drives?

For me data safety/integrity is most important factor when it comes to NAS and I would rather sacrifice some of the performance for safety.

But I don't wanna spend hours setting all up from hardware to servers and I'd gladly pay for a good solution that would fit my needs.

So I've been looking at DiskStation DS215j which provides pretty much anything I need, and since there is no ZFS support, I would probably end up using their Synology Hybrid RAID (SHR) which is open source and I like it ;P But I'm not completely convinced from what I read, that it is super safe and redundant with two drives.

You might suggest I buy 4 bay version, but according to THIS, with 3 drives the tolerable hard drive failures is 1 and even with 4 drivers it's only 2, so assuming there won't be two drivers failing at the same time... I'd don't see any point beside from increasing capacity. Also with 4 drives, its the twice the amount of money I'd have to spend which is a bit over my budget at the moment.

So how exactly does SHR create redundancy with two drivers?
Would in this case work similar to software RAID 1?
Any other suggestions?


As far as I understand this synology forum post: click me for I am a link. SHR is basically a parity based software RAID that can be dynamically enlarged and can operate with different drive sizes. It seems to partition the drives in such a way that it divides them into smaller chunks and calculates the parity for them.

SHR has the downside that if your NAS dies and you want to replace it and keep the drives you have to buy another Synology. SHR can only be read with a lot of effort in a normal PC (as one would do to reconstruct after the NAS died). And extending a RAID without a complete backup of the data is a sign of an overly courageous user.

SHR is nice and all. But not any more redundant than RAID 1 in a normal system. And Raid one can be read by any linux/windows as far as I know

That's exactly how I understood it too, so to me because it makes more 500GB chucks it therefore has more chunks of parity which IMHO makes it more safes than RAID1.

Well no, look at THIS it's pretty simple.

Well I feel safer with the idea how SHR works compared to RAID1 also RAID1 was never meant to be used as a backup solution, SHR was.

The chunks will probably be anything but 500 gigs since a 2TB Harddrive has a littlebit less than 2 Terabytes of space since drive manufacturers do calculate with 1000 megs in a Gig instead of 1024 Megs in a Gig. But that is a technicality. As far as I have understood it it will NOT provide more parity (1 drive redundancy means 1 drive's worth of parity).
Chunks are basically a renamed striping, does not make the system any more redundant i.e. secure. Striping or using chunks does improve read/write performance though (depending what files you access). RAID 1 and 5 are 1 drive redundant, as well as SHR with one drive redundancy. SHR with 2 redundant drives might be interesting if you have more than 4 drives of varying sizes.
Backup does also protect against user error an infection with some cryptlocker variant could encrypt the whole NAS drive and make the data unavailable. In this case SHR does not work as a backup, same goes for anything that includes a complete loss of the device. You could overwrite data by accident or the data could corrupt. So you would need ideally a cold backup you store at someone else's place.
The purpose for RAID is to provide large contingents of space by combining several disks. Risk of disk failure adds up. Let n be the number of disks and p the probability of a disk failure. If one would not employ parity the risk for data loss would be n * p. Risk for complete loss of data with one drive parity is p*p since two drive failure events would have to happen consecutively. RAID was indeed never made for backup purposes.

An utility to recover an SHR array is nice :)

Although I think you misunderstand the last sentence you quoted from me. I do not find SHR inherently dangerous since it provides redundancy. It is the ideal way if you start out in a multi bay system with two drives and want to extend the array later. But it is only as safe as a comparable RAID.

A friend of mine has also a Syno NAS. Rather two of them. One with 2x2 TB in RAID 1 and one with 2x1 in RAID 0 at his parents place. Every night the one at his parents place turns on and backs up his NAS at his place. A cold offsite backup. You could do a weekly sync with a external HDD to have a backup.

So by using one NAS with two drives would probably be then safer to have first drive act as live storage and second drive spinning up every few days only to back up the first one, than having RAID1 or SHR. As you said extending space is nice, but I'd rather have 1:1 backup than having this feature just so I can use it every few years.