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Comment by tgflynn | Hacker News Reader

What would be the advantage of having a SATA drive with DRAM instead of an SSD ?

The only potential advantage I see would be read and write speed, but aren't SSD's already fast enough to saturate the SATA bus ? Isn't that why NVME offers better performance than SATA ?

What is clear is that for equivalent capacity DRAM is much more expensive than an SSD.

My guess is that if no one is making a product like what you are asking about it's because it offers no real advantage over the much cheaper solution of adding another SSD.

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I guess normal RAM has pretty much infinite TBW compared to an SSD?

Some SATA SSDs have a cached mode where data gets put into normal RAM first, and then copied onto the SSD, but it's not really useful other than getting overinflated benchmarking numbers (things get messy if your system loses power before the data can transfer from RAM to the disk). Samsung calls it RAPID, the Crucial name is Momentum Cache

Some of the enterprise grade SATA SSD's have a fairly long lifespan which might be suitable. eg:

https://www.scorptec.com.au/product/hard-drives-&-ssds/solid...

That's a SATA drive rated for 5,600 TBW, and it's "maximum" rated write speed is 540MB/s.

To work out how long the drive should last at that maximum speed, let's do:

    540MB/s * 60 (seconds) * 60 (minutes) = 1,9444,000 MB per hour

Let's call it 2 TB per hour for simplicity.

We take the rated endurance of 5,600 TBW, and divide it by the above 2 TB/h, giving us "2,800" hours worth of write endurance.

At 24 hours in a day, that's 116 2/3 days worth of endurance at the maximum write speed the whole time.

Anyway, if the project in mind is going to run for less than ~120 days, it should be ok. Any other hardware/software issues aside that is.

Yeah, but with the difference in price you'd have to need to replace your SSD monthly for DRAM to come out cheaper and I highly doubt you'd be writing data that fast.

You can get ECC DDR3 ram for less than $1 per GB, and it doesn't wear out like SSDs do. Makes a lot of sense for a cheap cache disk. I can totally see using a ram disc like that for a zfs/bcachefs cache with a modern CPU that doesn't accept DDR3. Might make more sense to use PCI than SATA though.

A niche market, but I think there would be a market for ram drives like that, especially if they supported ECC memory. Lots of old server memory kicking around but if you want to actually use it your stuck with some abysmal CPU performance and a very high CPU power draw.

$1 per GB is $1000 per TB. I just bought a 1TB SSD for about $70. With such a large price difference you'd be better off buying a new SSD every year or more often, if you're worried about it wearing out.

At that price it's probably QLC, which will be too slow for a busy cache and will probably die in less than a year.

Nah, SSDs are going down nicely in price. You can get 1TB of TLC for less than $50.

As long as SATA is involved, a DRAM-Drive will not be faster than any decent SSD. For the SSD era, SATA is SLOW! That's why NVMe exists.

However, there reasonable reasons for wanting a DRAM-Drive.

> However, there reasonable reasons for wanting a DRAM-Drive.

What would those be ?

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