A friend of mine asked me to check over a spec of a Network Attached Storage (NAS) device he was buying for home. Mostly for media, backups, and general storage, and a light bit of work use. The spec he sent me bothered me a bit, and I gave it some thought before replying. The spec he’d chosen was:
-> 4 x 24TB Hard Drives
-> 32GB RAM
-> 10Gbe Upgrade
The main thing that bothered me was the upgrade to 10Gbe when only using 4 drives, shortly followed by the thought of 24TB drives simply being too big – my preference would be for more, smaller, drivers. Let’s say 8 x 12TB in this instance, to give the same available storage.
Let’s talk through this logic – I’d be interested in your views.
Recovery Time
This is probably my key observation when choosing larger drives – recovery times become uncomfortable. Let’s imagine you have a four drive NAS consisting of those 4 x 24TB drives, and one of them fails. You order another one, and pop in the new drive – all good, right? No data lost.
Well, consider how long it is going to take your NAS to rebuild that array back to the point that you have some high availability again. Assuming a fast spinning drive (I use these) they’ll average perhaps 125-150MB/s during a RAID rebuild. That means it’s going to take 2-3 days for your RAID to rebuild. During that time your data is exposed, and the performance of your NAS will be severely impacted.
Assuming smaller drives – say 12TB – that recovery will take just over a day. Still a significant amount of time.
Performance
Back in the day when I was designing large Microsoft Exchange on-premises stuff, we always went for more drives. This was for the performance requirements. You could often end up with a lot more physical space than you needed, in order to achieve the performance throughput you required. Let’s consider the 4 x 24TB and a 10Gbe upgrade – you’re not going to see all of the benefit of the 10Gbe upgrade. One of the fastest spinning drives I have is an Enterprise Toshiba 16TB drive – here is its performance in a Thunderbolt chassis:
Even with four of those in your NAS, you’re not going to see more than 600MB/s across them (assuming one is for parity/high availability), despite having approximately 800MB/s of raw performance. Sure, it’s still loads better than your 110MB/s you’d get over 1Gbe.
If however you used 8 x 12TB drives, assuming they offered similar 200MB/s performance per drive, you’d get approximately 1600MB/s (raw). Even dropping a single drive for parity gives you 1400MB/s, which means you would get the benefit of the 10Gbe upgrade. You should get your 1-1.1Gb/s performance.
High Availability
This ties back to the recovery time I mentioned earlier—more drives allow for more robust availability models. Take RAID 5, for example, or Synology’s equivalent, SHR. In a four-drive setup, one drive is dedicated to redundancy, meaning you can lose a single drive without losing data.
With an eight-drive setup, you could opt for RAID 6, which provides even greater protection. In this model, two drives are used for parity, allowing you to sustain up to two drive failures before data loss becomes a risk. Think of it this way: if a drive fails, you can replace it while still maintaining redundancy during the rebuild process. However, you wouldn’t want to wait for a second failure!
Anecdotally (though anecdotes ≠ data), drives purchased together often come from the same batch, and rebuilds place significant stress on remaining drives. I’ll let you draw your own conclusions on that…
Space Availability
Next, let’s look at usable space – best demonstrated below.
A 4 drive 24TB RAID 5 array gives you 72TB of available space, with 1 drive used for parity.
If you went with 8 x 12TB you’d better performance, and 84TB of usable space with a single drive for parity. I wouldn’t though, I would go with 8 x 12TB drives, in RAID 6 – in this model, you get the performance, the same usable space (72TB) as the 4 drive RAID 5, as well as two drives used for party.
Seems obvious to me!
Downsides of more drives?
It’s not all sunshine of course, there are some things to consider. Firstly – power consumption – 8 x 12TB drives will use about 1KwH of power a day (assuming about 5w/drive). 4 x 24TB will use half of that – a difference of 183KwH over a year. In real money – well, GBP – that’s about 50GBP in extra power used by the 8 drives over the 4, over the 4. Not so terrible when you look at it like that?
The other thing of course is that 8 bay NAS chassis cost more. A Synology DS923+ for example is about 500GBP, whereas an 8-bay Synology like the DS1821+ is nearer GBP950.
Let’s work out the costs, ignoring the RAM/10Gbe upgrade.
(For my American friends, in the UK our prices include VAT/Sales Tax – currently 20%).
Closer than you’d think isn’t it? The only downsides from what I can see are the increases in operational power costs, with the upsides being performance and data protection.
Summary
Personally, I’d go for more drives simply from the recovery time and data protection perspective – the increased performance is an added bonus.
I’d be interested in hearing your thoughts.
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