RAID 5 versus RAID 5 + Spare

Is RAID 5 + Spare preferable to simple RAID 5?  I understand that my storage space with four 4TB HD is reduced from approx 12TB to approx 9TB.  But my understanding is that I have the capability for a “hot spare” with RAID 5 + Spare and that I do not have a hot spare with simple RAID 5.  I also think I understand that simple RAID 5 is potentially problematic when one drive fails in that there is a finite risk of other drive failures when a failed drive is replaced and the array re-builds itself.

Do I have it correctly?  BTW, I am backing my array up to Elephant Drive.  If storage space is not an issue, i.e., 9TB is sufficient for current and intermediate term requirements, is RAID 5 + Spare a better configuration?

Before I go too much further with populating My Cloud.  I would prefer, if generally recommended, to re-configure now with a small amount of data than later with a much larger amount.


If your budget stretches enough then you can determine what you can afford to lose and what you can’t afford to lose. Get an external USB3 storage, set-up a copy job and make a back-up to your own USB3 storage. If you have another NAS then that could be used. If you know how to use the Linux commmand shell then you can use rsync directly to synchronise a back-up with the DL NAS.

I’m trying to figure out the advantage of a RAID5+spare. I’ve opted for for simple RAID5.

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Thanks much for your thoughts.  If you come up with additional thoughts regarding RAID5 + Spare, please post them.

IMHO, RAID5+Spare is only useful if the time it takes to get a replacement drive is measured in weeks.

Can you explain, please.  I probably don’t really understand RAID5 + Spare.

My original question was raised by some 2010-2012 discussions in other forums regarding the relative value of RAID6 versus RAID5 it which it seemed to be generally agreed that RAID 6 was superior because of apparent system failures in RAID5 systems during rebuilding after a drive failure.  Apparently because of the way RAID6 works such risks seen with RAID5 are obviated in drive failures in RAID6 systems.

I thought that perhaps RAID5 + spare might reduce such risks by automatically defaulting to the spare drive and automatically rebuilding in the face of a drive failure among the other 3 in a RAID5 + spare setup.

Since I mostly don’t know too much about RAID systems and concepts, I was hoping to optimize my system before getting too far down the road with a lot of data storage that would need to be restored if I switched from RAID 5 to RAID5 + Spare.

Thanks to anyone who is willing to educate.

I think you’re got the basics down correctly.

RAID5 (of four disks) – All four drives are running, reading, and writing.   If you lose a disk, it’ll go into “DEGRADED” mode, but all data is still available and Read/Write normally.

RAID5+Spare (of four disks) – THREE drives are running, reading, and writing in a 3-Drive RAID5 array.   If you lose a drive, and automatic rebuilds are enabled, the volume goes DEGRADED, the SPARE drive becomes ACTIVE and the array rebuilds into that spare drive.  It goes back to “Healthy” once the rebuild is completed.

When you replace the failed drive, it becomes the spare.

So, you’re sacrificing the space the 4th drive gives you, but you’re assured that the array is left in DEGRADED mode for as short a time as possible.

But if you have access to replacement drives fairly quickly (like within a few weeks) and you’re using RAID5 (with no spare), leaving it in DEGRADED mode for that long while you wait for a replacement isn’t a huge risk.  Especially since you’re also backing it up to an outside service.

RAID6 is different than RAID5 in that TWO drives are dedicated to Parity data, whereas in RAID5, only one is.

So if you’re doing RAID6, you’d have the same capacity is RAID5+SPARE, but it can survive the concurrent failure of TWO disks before the volume is unavailable.

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Thanks, very much.  That was a pretty lucid explanation.