低延迟DRAM何时才能普及？

低延迟DRAM(LLDRAM)的存在已将近10年之久了。然而，潜在用户对于采用这些产品仍然感到忧虑，因为他们都曾经历过多家供应商、产品互不相容、许多新兴业者加入竞逐但却也有更多厂商消失，以及供货来源有限等历史。

众多新加入的厂商有助于减轻业界对LLDRAM所感到的不确定性。但在该产品蓬勃发展之前，业界还必须就LLDRAM究竟可填补广大存储器领域中的哪一块达成共识。

在1975到1995年之间，有三种主要的非易失存储器技术占据电脑市场：DRAM、慢速SRAM(Slow SRAM)，以及快速SRAM(Fast SRAM)。从相对的每位元价格(PPB)观点来看，这些技术之间维持着稳定的关系。无论在任何时间点，使用者都可预期Slow SRAM的每位元价格大约会比通用型DRAM高出10倍。同样，Fast SRAM会再多出10倍，而芯片上存储器的成本则远不只另一个10倍。今天，我们拥有了新的非易失存储器产品，它提供更高的频宽介面，然而，当我们从延迟的角度来看，存储器的延迟特性与每位元成本间仍然存在着相对应的关系。

当问到“究竟何时DRAM才有出头日？”时，从历史的角度来看，答案是：“当它达到Slow SRAM的性价比时。”然而，怎样的成本才合理呢？

为了让存储器在特定的时间范围内加快技术进展，存储器芯片设计人员们开发出愈来愈小型的存储器模组，以缩短信号线，并减少寄生负载，从而让更多快速扩增的微小信号能在存储器单元上被检测到。较小的模组意味着更多的解码器、更多放大器，以及越来越多的驱动器、更多的路由，和更大的晶粒。LLDRAM以增加晶粒尺寸和产品成本为代价在延迟特性方面击败了通用型DRAM。这就是为何LLDRAM现在占据了在性价比方面一度以Slow SRAM的领域──尽管事实上二者都是以DRAM类型的位元单元所建构。

网络工程师绝对有权知道LLDRAM技术在网络市场的应用潜力。我们已经用20年的时间来证实RAM类型产品及其性价比在电脑市场上的可行性。我们也可以预期同样的事情也会发生在网络存储器市场。只要LLDRAM产品维持其产品优势，他们的生产成本就可以维持在传统Slow SRAM的每位元成本范围内，而这将使他们更加巩固市场。

因此，何时DRAM才能真正拨云见日？要等到它能像slow SRAM一样执行时吗？或是它的售价与Slow SRAM的相对每位元价格相同？两者都是。但也许对网络系统设计者来说，好消息是一些存储器供应商已经了解LLDRAM在需要高性能、高度混合、长使用寿命的网络存储器领域中所能提供的优势了。目前的LLDRAM供应商事实上也是专攻网络存储器的SRAM供应商。而未来，最好的发展或许是供应商能够像支持网络SRAM一样支持DRAM。

点击参考原文：When is a DRAM not a DRAM?

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When is a DRAM not a DRAM?

David Chapman

Low Latency DRAMs, of one form or another, have been on the scene for nearly ten years. Potential users have remained skittish about adopting them because of a sketchy history that has featured incompatible products from multiple vendor camps, the entry and disappearance of multiple vendors, and limited sources of supply.

The arrival of multiple new vendors in the market is helping to assuage some of the uncertainty associated with Low Latency DRAM. What remains before the technology can flourish is a general agreement of where LLDRAMs fit in the larger memory landscape.

From 1975 to 1995 there were three primary volatile discrete memory technologies that were mainly serving the computer market: DRAM, Slow SRAM, and Fast SRAM. From a relative price-per-bit (PPB) perspective, the relationship between these technologies remained constant. At any point in time a user could expect Slow SRAM to cost about 10X the price-per-bit of a commodity DRAM. The same number of Fast SRAM bits would cost approximately 10X more again, and on-die memory bits would cost another 10X beyond that. Today we have a new crop of volatile memory products that provide blindingly high bandwidth interfaces, but when viewed from a latency perspective, they still occupy the same relative latency and the same relative cost per bit niches where they have always been.

The answer to “When is a DRAM not a DRAM?” from a historical perspective is “When it sits in a slow SRAM price/performance niche.” But can those costs be justified?

To make a RAM faster in a given process technology, memory chip designers break the memory array into smaller and smaller blocks to shorten signal lines, reduce parasitic loading, and to allow more rapid amplification of the tiny signal being detected on the memory cells. Smaller blocks mean more decoders, more amplifiers, more drivers, more routing, and therefore larger die. Low Latency DRAMs beat commodity DRAM latency by increasing die size and product cost. That is why Low Latency DRAMs now occupy the price/performance niche once occupied by Slow SRAMs – despite the fact that they are built with DRAM-type bit cells.

Given recent history, networking designers have every right to wonder if Low Latency DRAM technology has staying power in the networking market. We have a two decade long existence proof for a RAM product type with their price/performance characteristic being viable in the computer market. We can expect the same thing to happen in the networking memory market. As long as Low Latency DRAM products stay within their mid-latency niche, the cost to produce them can remain within the traditional Slow SRAM cost-per-bit range, and the market for them will solidify.

So when is a DRAM not a DRAM? When it performs like a slow SRAM? When it sells for the same relative price-per-bit as Slow SRAMs? Yes, both. But perhaps the best news for networking system designers is the Low Latency DRAM banner has been taken up by the memory suppliers who understand how to serve the high performance, high mix, long life-cycle world of networking memory. The new LLDRAM vendors on the scene are SRAM vendors who specialize in networking memory. The best answer may be: When the vendors support it like a networking SRAM.