RAM模組比以往任何時候都便宜，所以為什麼我們不使用超高速的RAM庫來執行我們的整個作業系統呢？

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

問題

超級使用者閱讀器pkr298想知道為什麼我們沒有執行基於RAM的機器，而不是基於磁碟的機器。他寫道：

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?

從表面上看，他的調查是有道理的，但很明顯，我們並沒有沉浸在基於RAM的計算機構建中；背後的故事是什麼？

答案

SuperUser contributor Hennes提供了一些關於為什麼我們仍然使用基於磁碟的系統的見解：

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.)

如果您想閱讀更多關於RAM磁碟的內容，請檢視RAM磁碟解釋：它們是什麼以及為什麼您可能不應該使用它們。

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