How come it's recommended to have left over space on SSD but not RAM?

[kadify]

How come it's recommended to have extra space on your hard drive, whether it be a solid state or a disk hard drive, but RAM is used up to its full capacity (at least with the way macOS works)? Would the same rules not apply and it would be better to have left over RAM?

Also as a side note, my computer just started to double type the letter b while I wrote this post...=/

 

[Honza1]

OS can release memory when needed or swap it out to SSD/hard drive. There is nothing which can be done if your SSD or hard drive are full.
Releasing memory costs time and so it is done only when needed.

 

[kadify]

OS can release memory when needed or swap it out to SSD/hard drive. There is nothing which can be done if your SSD or hard drive are full.
Releasing memory costs time and so it is done only when needed.

Is that the only reason? It's just the way ram and the hard drive are implemented that causes the need to keep extra space on the hard drive but not the ram?

 

[Queen6]

Reducing the free space of an SSD to a very low percentage can effect the drives longevity, as the drive is forced to write to the same cells over and over, exactly how quantifiable this is who really knows.

System always pages data to the drive regardless of RAM, in some applications it's simply the way they are written. System will use as much RAM as possible to cache and accelerate applications, up to a point. Simple way to look at is the SSD/HDD acts and an overflow for RAM, unless and application explicitly requests to page data, in which case it pages to disk irrespective of the RAM installed.

The figure that's generally touted is 25%-30% free space.

Q-6

 

[magbarn]

SSDs significantly slow down when full. Their longevity goes down to as mentioned above, the same cells will be written to over and over which wears them out much fastef

 

[kadify]

Reducing the free space of an SSD to a very low percentage can effect the drives longevity, as the drive is forced to write to the same cells over and over, exactly how quantifiable this is who really knows.

System always pages data to the drive regardless of RAM, in some applications it's simply the way they are written. System will use as much RAM as possible to cache and accelerate applications, up to a point. Simple way to look at is the SSD/HDD acts and an overflow for RAM, unless and application explicitly requests to page data, in which case it pages to disk irrespective of the RAM installed.

The figure that's generally touted is 25%-30% free space.

Q-6

SSDs significantly slow down when full. Their longevity goes down to as mentioned above, the same cells will be written to over and over which wears them out much fastef

My question is why do the cells slow down from rewriting for the SSD but not RAM?

 

[Queen6]

RAM is volatile and is not physically written too, the SSD is NAND and is non-volatile requiring data to be physically written to it. Although modern SSD's are tremendously durable they still remain to have physical limitations. Reducing the free space to just a few percent, only serves to accelerate this process.

Bottom line is SSD's work best with adequate free space as it's a factor of their functionality, filling them to the max will only likely reduce performance and likely negatively impact longevity.

Q-6

 

[jerryk]

My question is why do the cells slow down from rewriting for the SSD but not RAM?

What do you mean by cells?

Both memory and disks get slower as the get full. SSDs (and other disks) have few places to write data to as they get full. Also, when you write data for a single file in hundred of locations these locations have to be found and the data loaded from each location when the file is read.

The main memory in a computer has the same issues, but is 100 times or more faster than even a fast SSD.

In addition, memory systems on modern processors also have high speed caches where heavily used pages of memory are stored. These caches are much faster than your 8GB or 16GB main memory. Around 100 time or more faster. And if you keep using a specific part of a program or file, this is where that data ends up residing.

So the net is that the SSD is 100 times or so slower than the memory, but memory is 100 times or more slower than cache.

 

[kadify]

What do you mean by cells?

Both memory and disks get slower as the get full. SSDs (and other disks) have few places to write data to as they get full. Also, when you write data for a single file in hundred of locations these locations have to be found and the data loaded from each location when the file is read.

The main memory in a computer has the same issues, but is 100 times or more faster than even a fast SSD.

In addition, memory systems on modern processors also have high speed caches where heavily used pages of memory are stored. These caches are much faster than your 8GB or 16GB main memory. Around 100 time or more faster. And if you keep using a specific part of a program or file, this is where that data ends up residing.

So the net is that the SSD is 100 times or so slower than the memory, but memory is 100 times or more slower than cache.

RAM is volatile and is not physically written too, the SSD is NAND and is non-volatile requiring data to be physically written to it. Although modern SSD's are tremendously durable they still remain to have physical limitations. Reducing the free space to just a few percent, only serves to accelerate this process.

Bottom line is SSD's work best with adequate free space as it's a factor of their functionality, filling them to the max will only likely reduce performance and likely negatively impact longevity.

Q-6

Interesting. I was referring to the cells Queen6 mentioned. So followup question. Why aren't SSD drives made out of this super fast memory that RAM is apparently made of? Is it because it's volatile? Maybe I'm missing something but I would think improving the speed of the hard drive to the point that we don't even need RAM would be beneficial.

 

[Queen6]

Interesting. I was referring to the cells Queen6 mentioned. So followup question. Why aren't SSD drives made out of this super fast memory that RAM is apparently made of? Is it because it's volatile? Maybe I'm missing something but I would think improving the speed of the hard drive to the point that we don't even need RAM would be beneficial.

Right now it's not technically possible, although progress is being made, rather in the opposite direction with non-volatile RAM. You might be interested in reading about Intel Optane and 3D Xpoint, equally still early days, expensive and niche...

Q-6

 

[jerryk]

Interesting. I was referring to the cells Queen6 mentioned. So followup question. Why aren't SSD drives made out of this super fast memory that RAM is apparently made of? Is it because it's volatile? Maybe I'm missing something but I would think improving the speed of the hard drive to the point that we don't even need RAM would be beneficial.

Cost is a big factor. But volatility is a primary reason. Cut the power and the memory gets cleared. Not a good feature on a drive.

Many systems engineering types and molecular material engineers are working on types of memory that would be static and faster than current ram. They have been at it for years, but nothing beyond the labs so far. Someday they will figure it out. Someday.

But even if they do, processor memory units need to be redone to take advantage of this instead of the page based systems we use now. And then of course, operating systems need to be redone to not be page-centric. And the files systems redone....

 

[NoBoMac]

JerryK beat me to it re: cost, constant power, reliability, etc.

Also, tying in to OSes need to be re-imagined to deal with a new paradigm of disk and RAM being same, you get an interesting problem of what happens re: you are getting close to filling the disk up and your "RAM" starts to disappear (probably block off n-GB of space for "RAM" and cannot be used by the filesystem). Security becomes interesting as well: your disk drive might now have sensitive information on it that was part of "RAM", need to have some type of encryption process in place.

RAM drives are nothing new. Been various incarnations of it over the years.

https://en.wikipedia.org/wiki/RAM_drive

Some companies (Oracle/Sun, IBM, and HP?) kinda do this with their database appliances: servers that have tons of memory in them to the point where they can place an entire database into memory and not have to go to disk (read: super fast database access).