Memory – Why Not Use SSD Space as RAM

Dell laptops with an mSATA SSD built in are in a RAID configuration enabling Intel's Smart Response tech to store a cache of the most used files on the SSD. This means faster boot times, application start times, etc. You can upgrade the built in mSATA to a bigger SSD, then go to the BIOS and change drive configuration to AHCI. You would have to re-install your OS but you can then install it on the SSD and use the hard disk as storage. This is my configuration and so far I have been happy with it with 4 second boot times to Windows 8. You really do not gain much if any battery life doing this, but the performance advantages in themselves are outstanding. I would caution that if you go this route, you should NOT install Intel's AHCI drivers as they are not compatible with the hard disk and would cause it to disappear for Windows (not the BIOS). Just use Microsoft's drivers and you'll be fine.

Windows memory management is a complex thing. As you see it has different behavior with different devices.

The different operating systems has different memory management.

Your question was very interesting. I am sharing a MSDN page which explains a part of the memory management in windows and more specifically "Mapped Files"

It's documentation for software developers, but Windows is software too.

One advantage to using MMF I/O is that the system performs all data transfers for it in 4K pages of data. Internally all pages of memory are managed by the virtual-memory manager (VMM). It decides when a page should be paged to disk, which pages are to be freed for use by other applications, and how many pages each application can have out of the entire allotment of physical memory. Since the VMM performs all disk I/O in the same manner—reading or writing memory one page at a time—it has been optimized to make it as fast as possible. Limiting the disk read and write instructions to sequences of 4K pages means that several smaller reads or writes are effectively cached into one larger operation, reducing the number of times the hard disk read/write head moves. Reading and writing pages of memory at a time is sometimes referred to as paging and is common to virtual-memory management operating systems.

Unfortunately we can't easy figure how Microsoft implements the Read/Write - it isn't open source.
But we know that it has very different situations:

From      To
==================
SSD       HDD
HDD       Busy SSD ??
NTFS      FAT
NTFS      ext4
Network   HDD
IDE0slave IDE0master // IDE cable support disk to disk transfer.
IDE       SATA // in this case you have separated device controllers.

You get the point... A hdd may be bussy, the file systems may be different (or may be the same)...

For example: dd command in linux copying data "byte by byte" - It's extremely fast (because the heads of both HDDs moving sync), but if the file systems are different (with different block sizes for example) - the copied data will not be readable because the file system has different structure.

We know the RAM is much much faster than HDD. So if we have to do some data parsing (to fit the output file system) it will be better to have this data in the RAM.

Also imagine you coping the file directly from-to.
What's happening if you overload the source with other data flows? What about the destination?
What if you almost doesn't have free RAM in this moment?
...

Only Microsoft engineers know.