Thoughts on a spare for a DAS / NAS?

I have a QNAP TR-004U config’d as RAID 5 with 4x8TB drives. Two of the drives are brand new this last fall. The other two are shucked externals with an estimated life of 2 and 3 years, respectively.

The drives I bought this last fall are on a decent price now, and I’m wondering whether it makes sense to get an extra while it’s a little cheaper. I could test it in an external enclosure if necessary - just wondering whether it makes sense to leave an unused drive on a desk for potentially 2-3 years waiting for another one to die, or whether y’all would wait to order until after I have a failure.


If the price is right I’d just replace the 2-3yr drives now, and use them for something else or store them.

FWIW I always kept a spare for RAID 5, especially when all the drives were purchased at the same time.

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You want to have a spare on hand. If your RAID fails, you are in a precarious situation until your array is rebuilt. You don’t want to have to shop for and buy one before that process can begin.

If you have enough drive bays, even better is to install an extra drive as a hot spare.

If your shucked drives are SMR, and the one on sale is CMR, then definitely replace them.

Agree that you should have a drive on hand to replace.

However, I no longer run a hot spare. A hot spare will
rebuild your array automagically if a drive fails. This is
great as a rebuild can take days (depending on size)

The down side is you could be rebuilding a bad volume
or pool. When I was doing a lot of traveling, I ran a hot
spare. Now that I am home, I did away with the hot spare,
as I will be available to check the volume/pool and start
the rebuild manually

If you live in a place, where it takes longer than 1-2 days, to get a replacement drive, it might be a good idea, to have a spare somewhere waiting for its turn.
If you have a look onto the steadily decreasing price per GB for HDD/SSD, it makes absolutely no sense to buy any drives to replace other drives maybe in 2-3 years.
Also what happens, if you need more storage a.k.a. bigger drives in a year, or two?

BTW, why do you think the drives will only last 2-3 years?
The majority of my drives within my Synology’s run now for 7-10 years, without any problems.
If I run into a problem with a drive, it was in the past always within the guarantee, so I get the replacements right away.

Ah, I didn’t mean I only expect them to last 2-3 years - I meant they were 2-3 years old when I shucked them and put them in the DAS.

“Steadily decreasing” isn’t my experience of drive prices lately. They’re up, down, and all over the map due to supply chain issues - and with inflation at current rates, I wouldn’t expect them to get substantially cheaper any time soon. Which is one of the many reasons why I’m contemplating a spare.

Replace both them and keep the used around as spares. Maybe one to clone your machine and one as real spare.

This way your NAS has all same generation and type drives, which is preferable.

So, if the drives fail, maybe due to a manufacturing error, they will all do at the same moment…?!

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The chance is statistically low as even if they are from the same series, they are not from the same production run. Think you are safe, Using same models and specs will only improve RAID performance.

Look at Backblaze for drive statistics and you will know their failure rate in % of large consumed quantities.

You can improve the RAID performance by using drives with the same size, and if you add similar drives, or larger ones.
And also if you use drives constructed for the use within a NAS.
There are even drives today where you have to (or at least are able to) consider the number of drives within your NAS to select the right model.
But it does, in my opinion, makes no differences, if those drives are from different companies, or with different ages.
I use NAS for almost 3 decades now, and had never an issue with different ages or manufactures of the drives.
And I also never heard about an official recommendation for that.

I would use the “older” drives until they get out of service, and then replace them by a new one.
There is no real sense, to let the “older” drives age outside of the NAS, to have them in “spare”.
If the newer drive fail, you put in the older one?
And then? Replace the older one again by a new bought newer one, as soon as you get it.
So your NAS is running in Rebuild-Mode twice for one failing drive? With the risk of a total loss, depending on your RAID, twice during that time?

Hot spares have their place.
For me, I would rather have a new drive ready to install, than a hot spare that has been running for potentially years and is now a ‘used’ drive.