Submitted by unlikemike123 t3_z6wvdf in explainlikeimfive
unlikemike123 OP t1_iy3i9q4 wrote
Reply to comment by unlikemike123 in eli5 How is computer memory deleted? by unlikemike123
O wait, that space is now available to be taken over by new data?
Hyperspacehost t1_iy3wgqy wrote
It’s worth thinking of computer memory less like a resource that’s used up, like water being scooped out of a bucket and drank, and more like sand in a sandbox.
Sometimes it’s being used for something meaningful, like being a castle or making the walls of a big hole or having letters written in it, and sometimes it just lying around doing nothing, but there’s not any more or less of it.
You might draw a line around a bit and say, “my castle is here, don’t use anything inside the line”, and at the end of the day you rub the line out and go, “I’m all done now, so do whatever”. Maybe you come back and the castle is still there or maybe someone has knocked it down and built something else with it.
SordidSimpleton t1_iy4461p wrote
This is a really cool analogy, props
Persist_and_Resist t1_iy3inlp wrote
Yep, that is what it means.
Neuro0Cancer t1_iy3iplc wrote
Exactly. The "free" memory is just the available memory, not the empty memory.
In fact memory that has been overwritten can still be recovered.
That's why there is software that "clean" your disk if you want, and it takes a while because basically what it does is just overwrite over and over again the disk until recovery is almost undoable.
[deleted] t1_iy41j00 wrote
[deleted]
AdmirableOstrich t1_iy470c2 wrote
There isn't really (official) publicly available software to do this, but depending on the storage technology you can often tell not only the current binary state of a memory bit but also the previous one.
As an example, we might say a bit is on if it is at 5V and off at 0V. However, real bits have voltages that vary quite a bit and we just threshold at (say) 2.5V. The actual voltages depend on (among other things) the previous state of the bit. If AB denotes a previous state A and a current state B, you might have something like:
00 -> 0.5V 10 -> 0.9V 01 -> 4.1V 11 -> 4.5V
Note that is doesn't quite work like this. The main point is that with dedicated hardware you can try to read data that has been lightly overwritten. This is why tools intended to purge data will overwrite many times with random bit sequences to minimize the chance of data recovery.
Alternative_Log3012 t1_iy4kkgi wrote
You won’t be able to do these sort of voltage reads in the standard OS the computer is running. I’m assuming you will need to remove the drive and put it into dedicated forensic recovery hardware. This could be even more difficult if your SSDs are soldered on like it most modern ultra books.
tim36272 t1_iy439pq wrote
Programs like Recuva do it for you. In the basic mode it is just reading all the entries that are marked as deleted.
Alternative_Log3012 t1_iy4k6mr wrote
This is not true / not what the OC is asking
dale_glass t1_iy3ircn wrote
Yup.
Also, fun fact, computers lie to users all the time. Eg, say you're playing a game and save. The game with very high likelihood will tell you it has saved well before the computer actually did it, and resume playing.
Why? Because people hate waiting, and the computer can actually do the writing to disk while you keep playing without making you wait for everything to be done.
There's many, many such optimizations.
ThatGenericName2 t1_iy3vbv5 wrote
There's also the lie in the other direction. While I don't think it's practiced anymore, sometimes software would "fake load" in that the program simply just waits for an extra bit of time because people would think that a computer didn't do something properly if it worked too fast.
goclimbarock007 t1_iy41vuk wrote
I read an anecdote about a programmer that wrote some sort of script that worked too fast and didn't look like it actually did anything. He added sleep counters and a progress bar to make it seem like it was doing something. A year later he "optimized" the program by cutting the sleep timers in half.
ClownfishSoup t1_iy4mkcj wrote
This weird thing happens with cake and cookie mixes as well. You can easily make a mix that just requires adding water and then baking, but people didn't believe they would be any good, so manufacturers changed the ingredients so that the end user has to add oil, milk and eggs to the mix. ie; people were suspicious that the mix was too easy, so they dumbed it down.
Brian57831 t1_iy41vtz wrote
Until you crash shortly after saving and the file shows it's corrupted.
DeHackEd t1_iy3mcrz wrote
People often ask why deleting files is faster than creating files. The answer is because the bulk of the file's data is NOT erased, because that's considered wasted effort. When the new file comes along it will write over the data, completely unaware of what was once there. So actually blanking out the old data doesn't really accomplish anything and the "delete" is really fast by just dealing with the file listing paperwork.
...unless there's privacy concerns that warrant actually clearing the data, specifically because preventing undelete from being possible is the goal. This is sometimes called "secure delete". But if you're uninstalling a video game, who cares?
EcchiOli t1_iy3oori wrote
Mostly useless remark, quoting you,
> People often ask why deleting files is faster than creating files
That's the case for the most common operating systems. However, sometimes, with server hardware + software, it's working differently and deletion can take a long time, as with linux's ext4. It's still relatively fast, but much slower than the others nonetheless.
kucksdorfs t1_iy3uze3 wrote
Think of it as a table of content in a book. So when you "delete' a file, you just remove the table of content chapter marker, but not the pages. Then, when you need to write more data, you "write" to the pages then update the table of content.
Things can get more complicated when it comes to flash storage (ssd/flash drive) but only when it comes to overwriting the pages.
eotty t1_iy45ump wrote
Imagine a binder with a table of contents, inside the binder there are pages, when deleting a file you just erase the page number, the page is still in the binder but you no longer know where to look. (If you want to find the pages again, there is special software, akin to looking through the binder page by page)
Some documents are several pages long, so when you later write something to the binder and replaces the pages, you might have page 4,7,9 of your document left, thats when it cannot be recovered anymore.
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