Prepare to have your minds blown, storage industry. 5 words: Client access Optane DIMM caching Super-fast storage array access looks to be coming, with persistent memory front-end caches in the accessing servers. Persistent memory (PMEM), also known as storage-class memory, is non-volatile solid-state storage in DIMM format, with DRAM-like access speeds but, hopefully, prices somewhere between DRAM and NAND. It’s used …
Forgive my ignorance but last time I commented on persistent memory I was downvoted horrendously.
Building super-fast persistent via a memory interface may be a recipe for speed; but it is also a recipe for hitherto unseen security disaster.
I'll be quite happy to let the technology evolve for 3-4 years, the inevitable chips to fall, and then, if at that point it proves reliable to start considering it. I would also hazard a guess that investment in better software and data structures would give a far better return than throwing inefficient processes at increasingly power-hungry hardware.
Can you explain why you think this is a recipe for disaster?
PMEM sits between traditional main memory and storage in terms of performance and cost (at least, hopefully it sits between them on cost...). The existing technologies have methods of addressing malware and other security issues and PMEM doesn't appear to present any new risks other than "things happen faster". If this is the argument, both scanning for malware AND malware storage access happen faster so I'm not convinced this argument is valid.
For pretty much the life of the CPU, I/O and in particular storage I/O has been the bottleneck for the amount of useful work that can be done by a system so I would expect it to be welcomed. The only potential downside for PMEM at present is pricing and ACTUAL performance * stares at Intel *
BIOS option: "Zero Memory on Boot".
Problem solved?
It's not like long-running OS (too long-running for my taste, this long-run thing seems to be fetish for some; reboot every week I say) and long-running applications with proper garbage collection are new territory.
the persistent memory acts as a front-end cache for the backend array and radically accelerates data access speed, except for cache misses of course.
And what about cache coherency?
Without it, this only works when you have a volume which is dedicated to a single host or VM. No global filesystems or shared data allowed.
Of course, block accesses are already cached in RAM. So this new technology only benefits you for the long tail of "warm" cache hits - you already have the "hottest" data in RAM.
"And what about cache coherency?"
The simplest example is a DBMS - where you currently have a large database that's much bigger than available RAM and you rely on SSDs for performance, you would now move to holding the entire DB in PMEM (assuming cost is similar to SSD but still much less than RAM) and either reducing RAM or gaining additional performance while using cheaper storage for your long term "permanent" storage needs (i.e. spinning rust or a disk/tape backup solution). All of your cache coherency issues are already managed by the DBMS so there should be minimal changes required with the exception of optimisations.
For other applications, the degree of difficulty in implementing PMEM and the subsequent benefits are left to the individual to judge as not everyone needs the additional performance.
'The use of host persistent memory is, for Peglar, a milestone on a longer journey to IO elimination: "Having said that, I look forward to further development of systems which actually help to, or completely eliminate IO, rather than just make it faster, i.e. with reduced latency, greater throughput, etc. by the use of persistent memory in true memory semantics, pure CPU load/store. This is the ultimate benefit of persistent memory."'
So the end of the Von Neumann architecture as we know it or marketing bafflegab bullshit?
Von Neumann didn't really have much to say about I/O or storage.
More relevant is what Thompson & Ritchie delivered with Unix. They decided everything (well most things) outside of memory looked like files or streams. Unix & other OSes took different kinds of incompatible block devices over which were built software stacks to make them look like uniform block devices (standardized on SCSI, and now also NMVe). Then they put a file system (there are now many varieties) over the block devices, and from there we get byte addressable files or streams.
Now we're entering an era where persistent memory is really going to change the way we deal with data; Peglar is absolutely right there. This PM stuff is fundamentally different from block based disks; it's byte addressable. We can do loads and stores on individual addresses, and we don't need the gymnastics & overhead of taking load/store memory semantics, building a layer that makes it look like a block device, then putting another layer over the top that makes it look like a stream or file. The difference is the persistence. Oh, and the performance. Although slower than DRAM it's much faster than flash.
If you want to know a lot more, go to the SNIA (Storage Networking Industry Association) site at https://www.snia.org/PM
In the meanwhile, the best way of using this is to employ it the way we know how right now. Making superfast low latency and high bandwdith storage devices & caches that can speed up operations by orders of magnitude is A Very Good Thing.
What he's talking about is the idea of writing to RAM like all programs do now along with that memory accessing operation doubling as an IO operation.
Say I have a program written in C. This program uses a memory pointer to write some value to an in-memory data structure. Now if you stop the program and restart it that same data structure now contains the updated value. No POSIX interface, no SCSI layer, no IO stack involved.
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Super-fast storage array access looks to be coming, with persistent memory front-end caches in the accessing servers.
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