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Comment by mritun | Hacker News Reader

Remember, even 1000 writes means roughly 3Y of full drive writes per day with the naïve wear leveling and the most brutal write load. For most write loads the drives made of this kind of NAND can be easily warranted for 3Y.

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This doesn’t account for the write amplification factor, which is quite unlikely to be 1 with a naive wear leveling implementation and the most brutal write workload.

note that there are data structures (eg b epsilon trees) that can guarantee a write amplification factor of log_b(disk size) for very large b (over 1000). this means that if you are willing to sacrifice a small amount of sequential read speed, you can guarantee write amplification of no more than 4 (with a few mb of RAM).

I've added [1] to my reading list. Do you have suggestions for others that relate specifically to WAF?

1. http://supertech.csail.mit.edu/papers/BenderFaJa15.pdf

The TLDR is that the design of b^e means that (other than the root node which you can store in RAM), you never need to write in chunks less than b^e (smaller writes get batched and pushed down the tree only when you have that much data to write to a child node). With the typical, e=1/2, you can set b=blocksize of SSD^2. This means that any small write will turn into 1 write per depth of the tree, and each of those writes will be writing a full block of data (so the maximal WAF is the depth of the tree). Also, for bigger writes, you can just write to the bottom directly, so this ends up working out such that for large sequential writes, you get a WAF of 1, and for small/random writes, you get a WAF of d.

1k writes sounds good enough for consumer hardware I suppose. As long as there's some some type of S.M.A.R.T. system to warn the user before it wears out.

usually it goes into read-only mode when it wears out, so there's no data loss.

that's fantastic actually! One problem might be that programs don't anticipate this behavior, like if it wears out while a filesystem is still syncing

all modern filesystems are either journalled or always consistent on disk, so shouldn't be a problem if it dies during

worst case you have to copy it to another disk and replay the journal

Yeah, 1K is still in the range of reasonable (if the wear leveling isn't terrible), another order of magnitude decrease though and we're getting on the edge of not.

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