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It is my understanding that it is the erases that wear out SSDs, not the writes themselves. Therefore, optimizing away the need for erases would be hugely beneficial from the point of view of drive manufacturers. Can I take it as a given that they do this?

I'd like you to assume that I'm writing directly to the disk and that there isn't a filesystem to mess things up.

dan_waterworth
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If there are empty pages in a block and the SSD wants to write to those pages it will not erase the block first. A SSD will only erase a block before a write if it cannot find any empty pages to write to because doing a full read-erase-write is very slow.

Besides, the wear-out from writing and erasing is about the same. Both involve pulling electrons through the oxide layer, just in different direction.

Also, the erased state for NAND is all 1. You then write 1 to 0. You need to erase the 0 to get it back to a 1.

Mr Alpha
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Unless I'm reading your question wrong I think you misunderstand how SSDs work.

SSDs are made up of large blocks (usually 512k), which are much larger than we are used to in a filesystem (usually 4k).

The Erase pass is necessary before anything can be written to the block unless the block is already empty.

So the problem with erases wearing out the disk is that if 4k of your 512k block is used, you must erase the whole 512k block and write the original 4k + anything else you are adding. This creates excessive wear and slows things down as instead of one "write" you need a "read-wipe-write" (known as "write amplification").

This is simplifying it a bit as the drive firmware does a lot of clever things to try and make sure the blocks are optimally filled e.g. it tries to keep lots of empty blocks to avoid slow writes.

Hope that helps/didn't confuse things further!

Stuart Brock
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  • I don't think I was very clear. What I'm trying to ask is: if only part of a block is empty (the part that I want to write to), will it still do an erase first? – dan_waterworth Nov 06 '12 at 18:21
  • Yes it will do an erase first. It can only write one full block at a time so it can't just write the bit you want to change. It needs to do the "read-erase-write" cycle. In reality it would use an empty block though (unless the disk didn't have any empty blocks). When the disk isn't busy it consolidates the partially written blocks together. It's a trade off between wear and performance to what extent this is done and is one of the big differences between drive firmwares. – Stuart Brock Apr 15 '13 at 09:57
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In the case that the SSD are using read-erase-write, it first read the content of the block, then erase it and write the new values. So it erase the entire block before writing, but it has saved the content for the next write operation. in some SSDs which are not read-erase-write, when you are writing, it will write on the new page (may be on the same block, then invalid the previous page). In this case, it erase the block only after making sure that every pages in the block are invalid or has been copied to another place.

NaSh
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