Comment by twotwotwo | Hacker News Reader

Comment by twotwotwo | Hacker News Reader

One potential application I briefly had hope for was really good power loss protection in front of a conventional Flash SSD. You only need a little compared to the overall SSD capacity to be able to correctly report the write was persisted, and it's always running, so there's less of a 'will PLP work when we really need it?' question. (Maybe there's some use as a read cache too? Host RAM's probably better for that, though.) It's going to be rewritten lots of times, but it's supposed to be ready for that.

It seems like there's a very small window, commercially, for new persistent memories. Flash throughput scales really cost-efficiently, and a lot is already built around dealing with the tens-of-microseconds latencies (or worse--networked block storage!). Read latencies you can cache your way out of, and writers can either accept commit latency or play it a little fast and loose (count a replicated write as safe enough or...just not be safe). You have to improve on Flash by enough to make it worth the leap while remaining cheaper than other approaches to the same problem, and you have to be confident enough in pulling it off to invest a ton up front. Not easy!

Any decent SSD has capacitor (enterprise) or battery backed (phones) DRAM. Therefore, a sync write is just “copy the data to an I/O buffer over PCIe”.

For databases, where you do lots of small scattered writes, and lots of small overwrites to the tail of the log, modern SSDs coalesce writes in that buffer, greatly reducing write wear, and allowing the effective write bandwidth to exceed the media write bandwidth.

These schemes are much less expensive than optane.

I have tried multiple enterprise SSD's, for sync writes. Nothing comes close to Optane Dimm, even Optane NVMe is 10x slower than PDIMMS.

https://forums.servethehome.com/index.php?threads/so-i-teste...

Yeah, but what percentage of the time was the CPU stuck blocked on the memory bus for the two workloads?

Also, how was it guaranteeing the page writes were atomic?

You are correct that writes are not guaranteed to be atomic. PostgreSQL has a feature to handle torn pages: full_page_writes = on, which is enabled by default. This means you do double write.

For RocksDB I believe torn pages are not a problem as SST files are immutable. But correct me if I am wrong.

> One potential application I briefly had hope for was really good power loss protection in front of a conventional Flash SSD.

That was never going to work out. Adding an entirely new kind of memory to your storage stack was never going to be easier or cheaper than adding a few large capacitors to the drive so it could save the contents of the DRAM that the SSD still needed whether or not there was Optane in the picture.

> It seems like there's a very small window, commercially, for new persistent memories. Flash throughput scales really cost-efficiently

Flash is no bueno for write-heavy workloads, and the random-access R/W performance is meh compared to Optane. MLC and SLC have better durability and performance, but still very mid.

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