All of us have heard of the not-so-new buzz surrounding the slew of new-er non-volatile memories (NVMs) that have either hit the market, or are expected to do so soon. The noteworthy among these are Phase-change memory (PCM) and an advancement of the Magnetoresistive Random Access Memory (MRAM) called Spin-Torque Transfer RAM (STT-RAM). PCM parts have started to trickle out as commercial products, while the viability of STT-RAM parts is still being demonstrated. Although, from the excitement about STT-RAM at the Non-Volatile Memories workshop this year, I am sure that we will be seeing more and more of it in the coming years.
Recently, the architecture community has been abuzz with proposals on applications of PCM, at different levels in the memory and storage hierarchy. But, the buzz notwithstanding, let's take a quick look at the PCM products that are presently available. Numonyx has 128 Mb serial and parallel parts on the market. While there has been talk of Samsung shipping a 512 Mb part, the part information is unavailable, as far as I know. Although, it does seem like Samsung is "shipping a little bit" of PCM parts. Where, to whom, and what for is still not very clear. But there have been reports of PCM parts actually being used in consumer electronics. From the current state of things, it seems like the consumer electronics industry (cameras, cell phones and the likes) is intent on using PCM as a plug-and-play replacement for Flash. Assuming that manufacturers come out with high density, low latency PCM parts, how are they best used in desktops and server-side machines?
Industry analysts believe that PCM is a good contender to replace Flash in the short term. Taking that thought a little further, IBM believes that PCM will have a big role to play in the server-side storage hierarchy, but again, as a replacement for Flash SSDs. I tend to agree with them, at least in the short run. Many research papers advocate that PCM will eventually serve as a DRAM replacement. Thus, there is little consensus on how and where PCM parts may fit in the memory/storage hierarchy.
From what I have learned so far, there are four main issues that will have to be handled before PCM is ready to be a main memory replacement - (i) endurance has to reach somewhere around that of DRAM, (ii) write energy and latency has to be reduced substantially, (iii) higher levels of the hierarchy have to be more effective at filtering out writes to the PCM level, and (iv) the recently discovered problem of resistance drift with multi-level cells has to be addressed. More recently, there have been concerns about scaling of PCM devices because of issues with increasing current densities and electromigration issues (Part 1, Part 2, Part 3). This, of course is assuming that all issues associated with wear-leveling have been worked out.
Apart from the issues with the devices themselves, has there been a general reluctance to shift to PCM? To quote Samsung's top honcho on the semiconductor side of the business "systems guys have been very reluctant to adopt the technology in mass quantities". The quote conveys very little discernible information, but, as a first guess, it might mean that the system software stack has to be extensively reworked to make PCM a reality.
There are many views on the future of PCM. There appears to be a slight disconnect between industrial trends and academic thrusts. The former seems to focus on using PCM as a permanent storage device, while the latter is attempting to move PCM higher up the hierarchy. A case can be made that more academic effort must be invested in developing PCM as a viable storage class memory.
Industry analysts believe that PCM is a good contender to replace Flash in the short term. Taking that thought a little further, IBM believes that PCM will have a big role to play in the server-side storage hierarchy, but again, as a replacement for Flash SSDs. I tend to agree with them, at least in the short run. Many research papers advocate that PCM will eventually serve as a DRAM replacement. Thus, there is little consensus on how and where PCM parts may fit in the memory/storage hierarchy.
From what I have learned so far, there are four main issues that will have to be handled before PCM is ready to be a main memory replacement - (i) endurance has to reach somewhere around that of DRAM, (ii) write energy and latency has to be reduced substantially, (iii) higher levels of the hierarchy have to be more effective at filtering out writes to the PCM level, and (iv) the recently discovered problem of resistance drift with multi-level cells has to be addressed. More recently, there have been concerns about scaling of PCM devices because of issues with increasing current densities and electromigration issues (Part 1, Part 2, Part 3). This, of course is assuming that all issues associated with wear-leveling have been worked out.
Apart from the issues with the devices themselves, has there been a general reluctance to shift to PCM? To quote Samsung's top honcho on the semiconductor side of the business "systems guys have been very reluctant to adopt the technology in mass quantities". The quote conveys very little discernible information, but, as a first guess, it might mean that the system software stack has to be extensively reworked to make PCM a reality.
There are many views on the future of PCM. There appears to be a slight disconnect between industrial trends and academic thrusts. The former seems to focus on using PCM as a permanent storage device, while the latter is attempting to move PCM higher up the hierarchy. A case can be made that more academic effort must be invested in developing PCM as a viable storage class memory.
