RAM模块比以往任何时候都便宜，所以为什么我们不使用超高速的RAM库来运行我们的整个操作系统呢？

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

问题

超级用户阅读器pkr298想知道为什么我们没有运行基于RAM的机器，而不是基于磁盘的机器。他写道：

RAM is cheap, and much faster than SSDs. It’s just volatile. So why don’t computers have a LOT of RAM, and on power up, load everything to the RAM from the hard drive/SSD and just run everything from there, assuming there’s no real need to persist anything outside of memory? Wouldn’t computers be much faster?

Of course, current operating system may not support this at all, but is there any reason RAM isn’t used this way?

从表面上看，他的调查是有道理的，但很明显，我们并没有沉浸在基于RAM的计算机构建中；背后的故事是什么？

答案

SuperUser contributor Hennes提供了一些关于为什么我们仍然使用基于磁盘的系统的见解：

There are a few reas*** RAM is not used that way:

1. Common desktop (DDR3) RAM is cheap, but not quite that cheap. Especially if you want to buy relatively large DIMMs.
2. RAM loses its contents when powered off. Thus you would need to reload the content at boot time. Say you use a SSD sized RAMDISK of 100GB, that means about two minutes delay while 100GB are copied from the disk.
3. RAM uses more power (say 2–3 Watt per DIMM, about the same as an idle SSD).
4. To use so much RAM, your motherboard will need a lot of DIMM sockets and the traces to them. Usually this is limited to six or less. (More board space means more costs, thus higher prices.)
5. Lastly, you will also need RAM to run your programs in, so you will need the normal RAM size to work in (e.g. 18GiB, and enough to store the data you expect to use).

Having said that: Yes, RAM disks do exist. Even as PCI board with DIMM sockets and as appliances for very high IOps. (Mostly used in corporate databases before SSD’s became an option). These things are not cheap though. Here are two examples of low end RAM disk cards which made it into production:

Note that there are way more ways of doing this than just by creating a RAM disk in the common work memory.

You can:

1. Use a dedicated physical drive for it with volatile (dynamic) memory. Either as an appliance, or with a SAS, SATA or PCI[e] interface.
2. You can do the same with battery backed storage (no need to copy initial data into it since it will keep its contents as long as the backup power stays valid).
3. You can use static RAMs rather then DRAMS (simpler, more expensive).
4. You can use flash or other permanent storage to keep all the data (Warning: flash usually has a limited number of write cycles). If you use flash as only storage then you just moved to SSDs. If you store everything in dynamic RAM and save to flash backup on power down then you went back to appliances.

I am sure there is way more to describe, from Amiga RAD: reset surviving RAM disks to IOPS, wear leveling and G-d knows what, However I will cut this short and only list one more item:

DDR3 (current DRAM) prices versus SSD prices:

• DDR3: € 10 per GiB, or € 10,000 per TiB
• SSDs: Significantly less. (About 1/4th to 1/10th.)

如果您想阅读更多关于RAM磁盘的内容，请查看RAM磁盘解释：它们是什么以及为什么您可能不应该使用它们。

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