Further to my previous discussion with you re my Synology DS1520+

Hi Leo,
I added two drives to my Synology DS1520+ (2xIronWolf Pro 12TB drives). So I’ve now got four identical drives in my NAS. My plan was to upgrade from RAID 1 to RAID 6 with a hot spare which I still have unopened in its box. Following YouTube tutorials I successfully upgraded to Raid 5 but when I went to take the second step and upgrade from RAID 5 to RAID 6 it’s asking me to insert the fifth drive that I’d intended to have as a hot spare. So it seems my choice is (a) to stay with RAID 5 and add the fifth drive as a hot spare or (b) go for RAID 6 without a hot spare. Which do you think is the better choice? TIA, Garry

I think the decision on whether to have a hot spare on not depends on part on your proximity to the Synology.

What I mean is: If the Synology is whether you live or work regularly, such that if you had a drive fail you would know it immediately (or nearly so), then the hot spare is less crucial in that you can replace the failed drive (fairly) quickly. At least in my area, I can get a new drive overnight from Amazon, sometimes same day if I order early enough in the day, put the drive in and be back to 1 drive protection.

If your Synology is located someplace else (say a friend or parent or child’s house) where you might not be able to replace a failed drive for a longer period of time, the hot spare allows the Synology to automatically rebuild the RAID protection using the hot spare - at which point you have RAID5 protection but no hot spare until you can replace the failed drive with a new hot spare.

RAID 6, by providing two drive failure tolerance is in some ways another way of having a “hot spare.” Compared to RAID 5 with a hot spare, RAID 6 is better in that if one drive failed, you still have a functioning RAID array with data protection (you can lose another drive without data lose) immediately, while with RAID 5 you don’t have safety against a second drive failure until the RAID5 array rebuilds with the hot spare, which depending on the disk size and amount of data can take many hours.

However, RAID6 is slower for writes than RAID 5, so if the highest write performance is a necessity for you, RAID 5 with a hot spare will be better in your case.

Two other concerns related to the drives themselves: If all of your drives were purchased at the same time from the same lot, then the probability of two drives failing within a short time of each other is, while still small, greater than if your drives were purchased at different times from different lots. RAID 6 can survive two drives loses without a rebuild in between; RAID 5 + hot spare cannot.

Secondly, if the Synology keeps a hot spare drive spun down until it goes into service, then the hot spare will essentially be a new drive when called into use. If the drive is kept spun up all the time even when not in use, however, then it is at the same point on its use/failure curve as the other drives, which means if all of your drives have been spinning for a few years and one days, you are effectively replacing it with a used drive, which is not what I would do in that case. I do not know if Synology keeps a hot spare spinning or not - some Google is in order there.

Hope this helps.


Go with the RAID6!
You never know which drive will fail, and with the RAID6 you have two who could fail, without immediate damage, so normally plenty of time to get a spare, and it also takes some time, to get the “Hot Spare” into operation, it is actually not so “hot”, because you need to copy the files anyway onto it.
If you are unlucky, you will get a second failure in between, and loose everything with a RAID5 at this point.

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Agreed. When you have a ton of data in your system, that could take many hours or days to rebuild, it’s nice when you learn the rebuild started while you were sleeping. Because you won’t be doing that again until everything is back to normal.:grinning:

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Not to pile one, but remember that RAID is about availability.
It’s not about speed (although you might get some depending on level)
or backup; but we all know that,

It takes a LONG time to rebuild a RAID array, always measured in hours,
and sometimes days! This is the purpose of a hot spare. It starts the
rebuild process “immediately” with no user intervention to restore availability.

This can be a wonderful thing, and Synology, like all vendors that provide
hot spare capability, keep the drive powered on and spinning in the chassis.

The downside is if your volume is corrupted! So yes, you are rebuilding, but you
have not addressed if/any of the root causes and now risk complete corruption.

When I was “traveling” I used a hot spare, now that I am “home” I have removed it.

As an aside, it is an interesting exercise to rebuild your array. Pull a drive out, put a
new one in, and see how long it takes to rebuild. If it were me, I would not use RAID 6
in a 5 bay unit, you are losing 2 bays to parity.

I would (and do) use RAID 5 with BTRFS.

Good points, @csf111, although I would add:

  1. RAID is about speed in the somewhat special case of RAID0, but that’s not what we are talking about here.
  2. I don’t know that the decision on RAID6 is necessarily dependent on the number of bays, as long as you have enough bays to support RAID6, of course. You can always increase available space with larger drives, and if your alternative is RAID6 vs RAID5 with a hot spare, then you are losing 2 drives to parity or to parity+hot spare either way.
  3. Other than RAID0, I agree that RAID is about availability. That being said, if there is enough of a performance hit with RAID6 vs RAID5 and you are using your NAS for active files, that can be enough to make the choice.

I personally have gone with RAID5 without a hot spare. My thinking is that a) the Synology is in my home, so I generally know about a failed drive pretty quickly, b) my Synology has a cloud backup so if tragedy strikes and I lose a second drive before I install a replacement and the system rebuilds, I do have a second tier backup, and c) it usually takes a day to get a new drive delivered, so my vulnerability window is generally about 2-3 days given detection, ordering receiving and installing a replacement drive, and rebuilding, which is tolerable given the cloud backup.

We are on the same page, RAID 5 without a Hot Spare,
for EXACTLY the same reasons.

I do think though, that bays (physical space) are a key
consideration for RAID type. Essentially RAID 5 uses 1
disk for parity (that’s a bay) RAID 6 uses 2.

The 1520+ has 5 bays, using Synology’s RAID calculator
and the above 12TB disks (60TB total)

RAID 5 - 48TB usable 12 parity (1 bay)
RAID 6 - 36TB usable 24TB parity (2 bays)

Of course this will be dependent on how much data
you want to have high availability, and perhaps your
use case can accommodate using 2/5 for “protection”.

To your point given the increased (insane) capacity
of today’s hard drives, perhaps it’s less of a concern.

I agree with you. Having a backup, and that’s what a second
Synology is for :slight_smile: I do have a spare disk on the shelf in the
event that I have to become the hot spare