Hacker News Reader

Comment by zrail | Hacker News Reader

I don't quite understand. Does this act like a stick of memory or an SSD? Is it just using the memory controller as a super fast parallel bus? If it does act like normal memory, what happens at reboot?

View on HN

Answer is there in FAQ. Written data is not lost on reboot, so basically you can mount it as a `ramdisk` partition and use that as data directory for your database. I have used this approach in past to speed up large test suite of a web application, except it required restoring from some sort of backup on reboot. Which won't be necessary in this case.

Does this mean you won't have traditional memory available at all (outside of L1-3 cache)? Or is it able to mix and match between real DDR3 memory and SSD?

It doesn't look exceptionally parallel -- the throughput figures are similar to those in PCIe SSDs. 880 MB/s doesn't come close to saturating even a PCIe bus (4 GB/s, for v2 8x), let alone DDR3 (13 GB/s per channel).

I agree, it's not that fast I have 1U servers with lots of RAM slots. Fitting in a RAID 10 is easy enough even in a "modest" 1U server with space for 8 DDR3 slots. We have servers with up to at least 16 DDR3 slots.

And in most servers these days, drives take up a massive amount of physical space. Being able to drop the drives and RAID controller has the potential of massively simplifying some server hardware.

Speculation: these SSD dimms will require more power and more cooling than the ordinary dimms.

> Does this act like a stick of memory or an SSD?

Like an SSD.

> Is it just using the memory controller as a super fast parallel bus?

Afaik, it's this.

Technically it acts like memory. The dev kit comes with a device driver that makes it look like a block device though, which would be analogous to something like NVMe.

IIRC a single DDR3 slot can address max 8GBytes. So the driver will have to map the 400GB Flash into this window.

DDR3 supports 8gbit chips and you can shove 16 onto one DIMM, so 16GB. But that leaves 3 address lines unused for column select. Theoretically they could make a 64GB or 128GB board. Also LRDIMMs can go up to at least 64GB.

DDR4 directly supports 512GB, and also throws away three potential column address bits.

Well at the interface level it has to look like memory since it's attached to a memory bus. But that does not mean it looks like a single large DIMM. There is no way in the DRAM protocol to delay a transaction while data is read from the flash chips into the buffer, so you need a protocol on top of the simple memory model.

"nvDIMMs" on the other hand do look like and are as fast as plain memory. Disadvantage is that they are not big.

Reply on news.ycombinator.com