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Consider a pretty fast Hard Disk.

I know many writes to a single disk will always be slower than many reads. For instance time taken to complete 100 write operations is much longer than time to perform 100 reads.

But

  1. How much slower is multiple writes over multiple reads.
  2. Define "many". How many simultaneous writes/reads would I have to perform to notice a difference.
  3. Are Writes just as fast as Reads on SDD ?
  4. Will a few 100 read operations on a disk bog it down ? (I'm sure not otherwise we won't be capable of running large databases )
Dhiwakar Ravikumar
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    Writes are much slower than reads on SSDs. On hard drives, there's not that much difference if the size of each write is large but can be significant if we're talking about a lot of small writes. – David Schwartz Nov 04 '14 at 10:40
  • Why are writes slower on SSDs ? How many small writes are we talking about ? What about 100s of reads ? At what point does the number of reads cause thrashing ? – Dhiwakar Ravikumar Nov 04 '14 at 11:04
  • Clarify your question. Are you asking about raw I/O, or about filesystem I/O. For a HDD, the difference between [raw write versus raw read](http://superuser.com/questions/350582/when-a-disk-read-or-disk-write-occurs-where-does-the-data-go/350592#350592) is not a significant amount of time. But if a filesystem is involved, then a write can be more complicated when new sectors/clusters have to be allocated. See http://superuser.com/questions/332787/cost-of-disc-i-o-read-versus-write/332800#332800 – sawdust Nov 04 '14 at 19:30
  • @DhiwakarRavikumar Writes are slower than reads on SSDs because writes often require an erase. If writes are rare, they'll be quite fast because they can be buffered (nobody needs to wait for them to complete) and the erasing can be done when the SSD isn't busy. But if you have sustained writes, then you'll eventually wind up waiting for erases, and erases are slow. – David Schwartz Nov 04 '14 at 21:50
  • Writes can be slower than reads but if you have a 500/512gb ssd write speeds are 450+ and reads are 500-550mb. While slower they are only a lot slower on smaller SSD. The newest samsung 850 w/3d transistors eliminates the slowness on ssd as small as 128gb. Large writes are more efficent than 1k read/writes. – cybernard Nov 05 '14 at 04:53

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