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I'm considering buying a new desktop computer. With both an SSD (probably a 1Tbyte model, such as Sandisk Extreme Pro M.2 NVMe 3D SSD - 1Tbyte) and some rotating hard disk.

My desktop computer at home stays powered on 24h/24 and run some Linux distro (usually Debian/Sid). I am even sometimes remotely accessing it from office. To summarize my activities, I am mostly running intensively compiler-like software.

Sometimes, I do have a program (perhaps my bismon in a few months) which runs (conceptually) for a long time (maybe weeks of cumulated CPU time). In practice, of course I am checkpointing and restarting it (but that is an implementation detail).

I will buy some (rotating) hard disk, which will be powered on 24h/24.

What kind of hard disk is more robust for such a purpose (I don't expect intensive disk accesses - since my applications are more CPU intensive than disk intensive and run on the SSD -, but I do expect them to happen once in a while regularly, and I want my disk to last 5 years, not just one.)

Some disks are branded as NAS disks (e.g. my favorite French shop has a NAS disk topic). Is it just a commercial brand, or does it means that such disks are more reliable?

When is it worth to put a "NAS disk" inside a desktop (powered 24/24)?

The NAS disk will mostly be used for backup purposes (e.g. a crontab job running every hour to backup a file tree from the SSD to the disk), and for files, such as downloaded ones, which are not often accessed; the main working disk -containing source code, object files, executables, and active data files- would be an SSD; That NAS disk would also have a swap partition, which in practice will never be used, because my future desktop would have a lot of RAM, at least 64Gbytes, and my programs usually won't use all of it.

Or should I ignore the "NAS" qualification and choose simply my disk on reliability (MTBF published by the disk maker) criteria? But of course I want a 7200RPM one.

More generally, what buying criteria should I have for a hard disk (of several terabytes, probably 4 to 8) which should last 5 years, in an always powered on desktop (well cooled).

PS. That disk won't be extremely active, like e.g. the disk of some RDBMS server would be.

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"NAS" or "24x7" labeled disks typically differ from their "desktop"-labeled siblings only in firmware and price - this goes so far, as there were some models where you could flash the "24x7" firmware to the "desktop" drives and they were happily working (early WD Blue vs. WD Red)

Those "NAS" disks (or rather firmware tweaks) claim to increase reliability of the drive and allow a 24x7 usage pattern - let's deconstruct those claims.

  • The 24x7 argument is IMHO fishy on first sight. What would be a non-24x7 usage pattern? Is a 14 hour development/debugging session out of warranty for a "Desktop" drive? Maybe a 12 hour weekend gaming session? They'd better write that on the box! And whatever thermal equilibrium is not reached within 8 hours is very likely to never be reached.

  • As for reliability: Since the MTBF ofcourse counts only against actual usage hours, a weekday 9-5 drive would need to last (24-0)/(17-9)*(7/5)=4.2 times as long as a "NAS" drive if measured in years. Please forgive me for laughing.

So what's really inside those tweaks?

The most important aspect is for those drives to fail more easily on transient errors. Yes, I mean that: "24x7" drives will very likely run in a RAID environment, where a slowly dying drive massivly increasing latency is the much bigger problem than a few sector reallocations or even a clean death. This means, that it actually makes sense to do lesser retries on transient errors and sooner pass them on to the host's software stack, where it can be handled in a way making sense. A degraded array is better than an unusably slow one.

The next aspect is power management: While NASes tend to run 24x7, they do not have a flat usage pattern over those 24 hours. They peak and idle with their user's access patterns. So reducing the power consumption of such a drive while it is close to or completly idle does make an appreciable input to the total power consumption of the NAS.

To summarize:

  • Forget the "24x7" stuff, it's marketing only
  • If you use RAID (other than 0) and can not easily tolerate a dying disk slowing everything down to a crawl do use NAS-firmwared drives
  • If you want your chances to recover data from a dying drive to be as high as possible, do not use them
  • Weigh those last two properties aganist your budget and the prices asked

And one more: Take a look at the propabilites of the drive being replaced for a defect vs. being replaced for being too slow or too small.

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I use a 5TB usb disk for my backups. I generally have it unmounted and have set to go into sleep mode when it is unmounted. My backup scripts mount it when required and unmount it when they're finished. No, it's not the fastest solution, but it is relatively inexpensive and the disk gets very little wear.

  • You should mention another positive aspect of the unmounted drive: It is safe against most human errors and most malware – Eugen Rieck Jan 7 at 8:17
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NAS disks are primarily aimed for usage in NAS'. I know, doesn't help you that much. A NAS is Network Attached Storage. It's a device with one or more disk bays (to enable redundancy) that's connected to the network. Normally used to have access to shared resources from multiple network devices, for example for video files that can be accessed by the smart TV, the computer or the smartphone. You will probably get the idea if I stop my explanation here. ;)

So hard drives for NAS purposes are quite reliable (24/7 certified) but also certainly not the fastest ones in terms of reaction times. Focus is on storage and data transfer, but not random read and write operations like a drive you would want in your computer to put your operating system on or something like that.

So as a NAS can also splendidly be used for backup operations, I don't see a problem with a NAS HDD in your PC for backupping. Just let me add, that your scenario doesn't quite sound like a real backup if you just copy data from one hard drive to another while both hard drives are in the same computer. But surely you know that.

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