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0 pointsdevit7y ago0 comments

Is there are a reason to not just use DRAM along with a battery to achieve the same persistence but as fast as DRAM?

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8 comments · 4 top-level

wtallis7y ago· 2 in thread

There are NVDIMMs that have DRAM and a matching quantity of NAND flash memory to save the contents to in the event of a power failure. They require an external capacitor module and are limited in data capacity by how much DRAM you can fit on the module. You can fit far more 3D XPoint memory on a module than DRAM, and it doesn't require the external capacitors to achieve persistence, and it should be significantly cheaper on a per-GB basis.

cdoxsey7y ago

Just to reiterate the point... this is an instance with terabytes of near-memory-speed storage.

If persistent memory pans out as a technology it will completely upend the way we think about building software and the cost tradeoffs of hardware. (as much as or more so than the transition from spinning disks to ssds)

Dylan168077y ago

How do you define "pans out"? What performance and price differences between it, flash, and DRAM do you have in mind?

Because I'll keep reminding people that putting a DRAM cache in front of some flash can very closely approximate a large persistent memory. If people wanted to build software for that kind of system, they could do it today. The hardware is not the blocker.

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manigandham7y ago· 2 in thread

The reason is the battery. These devices can be powered off and save state, like an SSD.

piotrkaminski7y ago

But in the context of the OP, presumably devices in a data center would never be powered down on purpose to save energy? In which case it seems that battery-backup DRAM would work just as well for this use case, and be both cheaper and faster.

jordanthoms7y ago

Optane is in between DRAM and flash in terms of cost (and performance) - it's also denser, so you can fit much more storage on each DIMM slot.

juancampa7y ago

Probably because systems that passively do their job tend to be preferred.

Also a battery would only last so long. IIRC DRAM needs to be constantly refreshed, so, it would be a trade-off between capacity and duration.

Optane seems[1] to be 20~30X slower than DRAM but 4~10X faster than server SSDs

[1]: https://superuser.com/a/1195674/187732

zokier7y ago

DRAM can not be simply naively battery-backed; it needs active refreshing. And as memory controllers reside in CPUs these days, that would mean keeping the CPU powered up.

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