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Comment by JoeAltmaier | Hacker News Reader

Can't find corroboration for the assertion 'SSDs are an awful lot better at sequential writes than random writes'.

Doesn't make sense at first glance. There's no head to move, as in an old-style hard drive. What else could make random write take longer on an SSD?

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The main problem is that random writes tend to be smaller than the NAND flash erase block size, which is in the range of several MB.

You can check literally any SSD benchmark that tests both random and sequential IO. They're both vastly better than a mechanical hard drive, but sequential IO is still faster than random IO.

Seems to be the case, to some degree. My SSD is 8% slower doing random writes. I guess your mileage may vary.

The key aspect is that such memory generally works on a "block" level so making any smaller-than-block write on a SSD requires reading a whole block (which can be quite large), erasing that whole block and then writing back the whole modified block; as you physically can't toggle a bit without erasing the whole block first.

So if large sequential writes mean that you only write full whole blocks, that can be done much faster than writing the same data in random order.

In practice, flash based SSDs basically never do a full read-modify-write cycle to do an in-place update of an entire erase block. They just write the new data elsewhere and keep track of the fragmentation (consequently, sequential reads of data that wasn't written sequentially may not be as fast as sequential reads of data that was written sequentially).

RMW cycles (though not in-place) are common for writes smaller than a NAND page (eg. 16kB) and basically unavoidable for writes smaller than the FTL's 4kB granularity

Long story short: changing a bit from 1 to 0 is really easy, but changing it from 0 to 1 is quite difficult and requires an expensive erase operation. The erase works on a full 4k block, so writing a random byte means reading 4k to a buffer, changing one byte, erasing the page, and writing back 4k. Sequential writing means erasing the page once, and writing the bytes in known-pre-erased sections.

Any modern (last few decades) SSD has a lot of sauce on top of it to reduce this penalty, but random writes are still a fair bit slower - especially once the buffer of pre-prepared replacement pages runs out. Sequential access is also just a lot easier to predict, so the SSD can do a lot of speculative work to speed it up even more.

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