Storage misconceptions - hard drives and SSDs

Here are a few misconceptions about storage which I have come across over the years:

  • the disk plates are moving faster on the inside that is why it is faster to read.
    • NO: it actually is more efficient to read the data on the outside part of the platter because the head can read more sectors without moving
  • the heads are reading at the same time.
    • NO: only one head is reading at a time – they get on and off as one head reads and the rest are off.
  • What happens when one disk platter is damaged?
    • You can read and recover some of the data. But what happens when one chip of SSD is damaged? There is no way to restore any data from the rest of the storage.
  • SSD vendors say that the SSDs are safe and there is an algorithm to mark cells as unusable as they get worn.
    • But if this is the case, then the capacity also diminishes as cells are marked as unusable.
  • fragmentation does not matter for SSDs.
    • NO: Fragmentation does not matter for read performance (even though fragmentation causes extra reads, and as we know all drives have limited IOPS), but affects the capacity of the available disk space. Fragmentation potentially hinders write performance and contributes to extra wear of the disk.
  • When reads and writes are related to CPU usage and memory, how much of a bottleneck does the fast IO cause in other areas?
    • In reality, when we ultimately speed up one part of our system, then we certainly get performance problems with the other systems. If we have very fast disk access, this still means that we have limitations in memory speed, CPU speed and Front Bus speed. In other words, solving one problem introduces several other.



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