Those of you who’ve followed us here at in-koherence know that we’ve taken a skeptical eye to using SSDs in DVRs, although things started to turn around a couple years ago. Still, even with improved endurance and falling prices the good old hard drive reigned supreme. While they eventually suffer mechanical failure (usually degradation of the platters) after a time, they tend not to fail just because you’ve written a certain amount of data to them. In regular use the drive will suffer mechanical failure long before then. This isn’t the case with SSDs. The NAND flash cells can only be written a given number of times, and although most employ sophisticated algorithms to optimize the wear of those flash cells, you need to pay attention to a drive’s endurance rating.
But something interesting has happened in the storage market – high endurance and somewhat affordable SSDs have started to appear – thanks to the chia cryptocurrency of all things. And a reasonably-priced high-endurance SSD is just what’s needed for DVRs.
The Chia Cryptocurrency
For those of you who haven’t been following cryptocurrencies, chia is a relatively new entrant which launched in May 2021. While previous cryptocurrencies tended to be very compute intensive – consuming vast quantities of electricity to mine on CPUs and GPUs – chia uses a greener “proof of space and time”. That space part is the key to why chia has spawned the new breed of high-endurance SSDs.
Chia “farming” involves two phases plotting and farming, each with distinct storage requirements. One starts by creating plots, which then sit around and are harvested. The harvesting process is not at all CPU intensive but wants a large amount of storage to store a bunch of 108.8 GB plots. (Please note that throughout this post we’re using GB = 1,000,000,000 bytes, as is common in drive literature, and similar for MB and TB.) While a large amount of storage is required, harvesting is neither read nor write intensive. Slower, energy-efficient hard drives make perfect harvester storage.
The chia plotting process in contrast is moderately CPU intensive but extremely write intensive on storage. While a chia plot is only 108.8 GB, the process of creating that plot requires about 1.4 TB of writes to temporary storage. If you were to take a consumer SSD with an endurance on the high side such as the TeamGroup GX2 2 TB SSD with its ~1600 TBW endurance, the drive might wear out after creating ~1200 plots. A NUC 8i7 can generate ~10 plots/day, so you’re looking at wearing the drive out in about 4 months, less if you have a heftier plotter! (Note: TBW is calculated according to JEDEC specs which generally assumes conditions worse than those usually encountered in chia mining. You might get two or even three times the number of plots out of a drive if you’re lucky. But let’s assume the advertised TBW for now…)
Enter a new class of SSDs designed for chia plotting. SSDs certainly exist that have high endurance ratings, but they’ve historically been targeted towards data centers. And in addition to high endurances they have additional enterprise features such as loss-of-power protection and enhanced IOPs which contribute to their higher price tags. The ideal drive for chia plotting – and DVRs – would have a data center class endurance rating without the enterprise class features, and a price tag closer to consumer grade SSDs. TeamGroup has introduced just such a line of SSDs – the T-Create Expert PCIe 3.0 SSDs. The 2 TB version has an amazing 12,000 TBW endurance. It’s price tag is still a bit hefty at a rumored $800. But if you compare it to the GX2, the T-Create Expert has a 7.5x endurance but only a ~4x price tag, so is a worthwhile investment for chia plotters. In contrast one of the recommended plotting drives is the enterprise-class Intel D7-P5510, which carries a 7168 TBW rating at a cost of $1500.
What Endurance Do DVRs Need?
DVRs tend to have a write-heavy workload, recording more shows than users watch – either because of an always-on live cache (ala TiVo), or because users set up lots of season passes and then don’t find time to watch all those shows.
But just how much endurance do you need? Let’s assume the following:
- Each tuner is recording constantly into a live cache, just as a TiVo DVR does.
- There is one channel per emission, meaning it uses close to the full 19.4 Mbit/s in an ATSC 1.0 emission. While this was more common in the early days of ATSC, there are still cases today where a single HD channel dominates an emission. For example until mid 2019 the SF Bay Area CBS channel KPIX-DT was allocated ~ 15.5 Mbit/s for its main HD channel and ~2.8 Mbit/s for the Start TV subchannel.
- The DVR has eight tuners.
- The write amplification factor is close to 1.0.
With these assumptions the DVR will be writing about 1676 GB per day. So in a year it’ll write about 612 TB. Over a five year period you’ll generate about 3,000 TBW. This is well within the comfort zone of even the 1 TB TeamGroup T-Create Expert with its 6,000 TBW. The 2 TB version’s 12,000 TBW would even be suitable for the platform powering SFBayATSC, which monitors 15 ATSC emissions and generates about 1000 TBW/year.
The more attentive of you might think that a 4 TB TeamGroup GX2 with an endurance of 3200 TBW would also meet the needs of our hypothetical 8-Tuner DVR. And this is certainly true – in this case you’re achieving the desired TBW by overprovisioning (not that 4 TB of DVR storage is a bad thing, but its probably more than most users need). Unfortunately at the time of writing a 4 TB TeamGroup GX2 doesn’t appear to exist. The closest we could find is the 4 TB Samsung 860 Pro with an endurance of 4,800 TBW. It’s currently listed at $700 on Amazon, or close to twice the cost of the 1 TB T-Create ($400).
(When shopping around take a careful look at endurances – they can vary greatly. For example the Western Digital WDS400TB0A 4TB SSD has a 600 TBW rating, while the Samsung 4TB 860 EVO a 2,400 TBW.)
There’s a lot to like about using an SSD in a DVR. They consume less power and are silent. And at long last SSDs appropriate for DVRs are arriving at a palpable price point. Hard drives still reign when it comes to sheer capacity, but we’re approaching the point where SSDs are viable for even write-intensive workloads.
Now if only those chia farmers would stop hoarding every SSD and hard drive they can set their eyes on…