Wipe/Erase Free Space on a SSD

[Kevin41269]

When using Wipe/Erase Free Space does "FF" or "11" get written to the drive or is it "00"? The reason I ask is when using a HDD the "00" means the space is free, however with a SSD "FF" or "11" in the space means it is free. This comes into play when writing to the SSD. When writing the SSD first reads the space to be rewritten then writes back to it. If the space is marked "FF" then is just writes which means faster transfer times. It also means garbage collection must do less work and the SSD last longer.

[Nergal]

Don't use wipefreespace on SSD the writes will just degrade the lifespan of your device. Ccleaner's WFS writes a bunch of files filling the space up completely then deletes those files

[Augeas]

I think we should step back a little. The whole concept of wiping free space on an SSD is fraught with well, it's just fraught.

 

I started to describe what I think happens when CCleaner attempts a WFS on an SSD, but it became too complex. So I'll say that WFS is not a good idea to run on an SSD. The files that CCleaner creates to overwrite free space are deleted in the MFT at the end of the WFS run, but not flagged as deleted to the SSD, unless CCleaner asks the O/S to issue a TRIM command (which I doubt). So the SSD doesn't know that it can release any of the overwritten pages for GC.

 

To answer your specific questions CCleaner writes files of zeroes to overwrite free space. In an SSD an empty page is - conceptually at least - full of FF's. However empty pages are unmapped from the lba page mapping tables, so a request to read an empty page will get a default page of zeroes sent to the file system.

 

Is the SSD controller clever enough to recognise that CCleaner is about to write a page of zeroes, so it just unmaps the lba? I don't know, possibly some are. I doubt whether CCleaner or any application writing a page of FF's is equivalent to erasing a page. If it were then someone could make a fortune. A file full of FF's is a valid file, so can't just be handed over to the GC. A file has to be deleted and a TRIM command issued by the O/S for the pages to be released to the GC, otherwise they remain valid until the lba is reused in the future.

 

Thinking further about this, if the answer to the first sentence in the previous paragraph is yes, and there are some indications that it is, then running a WFS would be a great idea, as writing zeroes would flag the page as free for GC and minimal data would be written to the SSD. That reverses all that I've said - just be sure to use one pass of zeroes only!