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I am new to the hardware side of things. I run a few machines which have 400GB+ SSD's and 32GB RAM. I have been thinking about going up to 64GB RAM, however, I was thinking, since SSD's are solid-state like RAM, can't my extra space be used as RAM?

If I do this, will the extra RAM (from disk space) be significantly less efficient than DDR3 RAM?

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Isn't that basically what "swap"/"pagefile" functions in modern OSes are doing? –  grawity Jul 10 '13 at 9:44
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Because SSD have limited writes, and your system makes hundreds of writes to your memory every hour, which means a SSD device would have a lifespan of a few days at that rate. Plus in terms of pure speed SSD is extremely slow compared to memory. Random Access Memory does not store the values after the power has been turned off, NAND the memory sed in SSD hardware does. NAND would make horrible Random Access Memory for a lot of reasons other then speed. –  Ramhound Jul 10 '13 at 10:53
    
If it were that simple, wouldnt everyone be doing it? –  Keltari Aug 4 '14 at 16:47

4 Answers 4

Firstly, RAM is still significantly faster than both your regular 6gb/s SATA or even the newer PCI-e based solutions. RAM is also designed to be written and erased repeatedly, at the cost of volatility. RAM generally dosen't wear out due to use regular use - tho, of course, it can fail like any component.

While the lifespans of SSDs have gotten much better, SSDs do wear out. They're absolutely brilliant for non volatile use, but if you wrote and overwrote NAND (which SSDs contain) like you do RAM, it would wear out.

Both are really optimised for different things, and you're better off having enough RAM (and using SSDs or spinny hard drives for paging out) than compromising on enough RAM for the task.

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As grawity suggested you already have the swap/page file performing this task. Now even an SSD is much more slower compared to DDR3. SSDs can deliver up to about 654MB/s while 1333MHz DDR3 in dual-channel mode can deliver up to 21.3GB/s (21 332MB/s).

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Yes, it will be significantly less efficient than DDR3 RAM.

  1. SSD will wear off quickly if used as RAM (frequent writes). So it will only be effective for 2 months or so, after that it will surely die. (So instead of 10 years of life ... it will live for about 10 weeks.)
  2. SSD is a disk device. CPUs can only pre-load data into its cache from RAM. If it will be on an SSD, it must be first loaded into RAM... Accessing the disk (even very-fast SSD) is around 100 times slower than accessing RAM. See benchmarks of HDD, SSD and RAMDISK (ramdisks on DDR3 have more than 3000 MB / sec , and less than 0.1 milisecond wait time for access. So, clearly: SSD cannot compete with speed of RAM).
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The key reason why you can not use an SSD as ram is because it is connected to the computer as if it were a disk drive, rather than ram. That is to say, that the processor can not directly address the memory in the SSD but instead has to hand a block of ram to the SATA controller and ask it to transfer data between that ram and an area in the drive.

The new NVM Express interface is poised to change this. It allows the CPU to map swaths of the SSD directly into its memory space and use it like RAM instead of issuing IO requests to transfer between RAM and the SSD. This has the potential significantly speed up access to the SSD while using less RAM as it is no longer required to cache the data while the CPU accesses it. This is currently an area of active development in the linux kernel.

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The first paragraph is basically the correct answer to the immediate question. Speed and wear on drive mentioned by others are secondary issues - even if SSDs were just as fast and resilient as RAM, currently we still could not use it as RAM. –  mtone Apr 12 at 16:53

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