Comment by electroly | Hacker News Reader

Comment by electroly | Hacker News Reader

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Wear leveling prevents anyone from writing to specific blocks. It decouples the OS's block numbering from the physical blocks. That indirection allows it to distribute writes evenly across the drive even if the OS is writing to consecutive blocks (or the same block).

This behavior is fundamental; it's what wear leveling is and why it exists. Wear leveling exists to prevent you from writing to the same block repeatedly, which is exactly what you're trying to do with shred.

I'm not sure what other source you're looking for, but I hope you find it. Perhaps, at least, the number of people "on StackExchange, ServerFault and Reddit" and now also HackerNews telling you the same thing is sufficient evidence to stop spreading the idea that shred might work on SSDs. Do what you want on your own systems, of course, but this is dangerously misleading guidance to be offering on an open forum.

So my concern with following this logic is that such tools that are writing to specific block locations would then be wiping unrelated inodes depending on the low level logic. I tried emailing Colin but his last known email is invalid now. He may be working for the NSA so I probably won't be able to reach him. Some people there are responsive, some are not and a few are on here.

So even if wear leveling prevents overwriting a file then such tools should in theory be a risk of data corruption. If this is the case then the tool should be updated to detect if the target is an SSD and abort with a scary message. Perhaps another route is to reach out to the coreutils team.

> writing to specific block locations would then be wiping unrelated inodes depending on the low level logic

It won't wipe unrelated files as far as the file system sees it, but it might overwrite some previously discarded internal blocks. "inodes" is a too high-level concept in this context.

You can read more about it on wikipedia which might be an authorative enough source for you? https://en.wikipedia.org/wiki/Wear_leveling

Also note that most ssds actually have more internal blocks available than what they present to the host device so that they can have a "cache" to be able to move things around internally, and also so they can mark certain positions as "bad" when writes to one internal block start failing and still operate properly.

Does that mean that partitions on an SSD therefore do not affect the long term wear on the SSD?

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