SSDs and RAM

Discussion in 'Mac Basics and Help' started by Thiol, May 19, 2009.

  1. Thiol macrumors 6502a

    Thiol

    Joined:
    Jan 26, 2008
    #1
    Hi all. So, I know that RAM amount is one of the first bottlenecks that any computer runs into. Running out of free RAM forces writing to a disk cache that is much slower. Here's my question, which I guess is sort of theoretical: Are page outs a problem with SSDs? If one has an SSD, that is almost certainly faster than a hard drive with most types of reads and writes (I know, but not all). I guess the connected question is: What type of reads and writes are involved in page outs? Are they small or large, random or sequential?
     
  2. Makosuke macrumors 603

    Joined:
    Aug 15, 2001
    Location:
    The Cool Part of CA, USA
    #2
    The non-theoretical answer is "yes, they are still a problem." While an SSD is significantly faster than a spinning drive for most operations, it's nowhere near as fast as RAM, and so will still be a huge bottleneck if you're doing significant pageouts to disk. The exact behavior will vary depending on what the system is doing, but my not-particularly-educated guess is that pageouts do lean toward random medium-sized transactions.

    This would give SSDs a theoretical additional advantage over rotating storage, but on the other hand you could also get into issues where the heavy read-write cycles on the flash disk could cause performance degradation over time (due to the way flash memory deals with re-writes of used space). Either way, the SSD would still be a major bottleneck.

    Keep in mind that even if a SSD could saturate a SATAII bus you'd be looking at about 300MB/s throughput; that's in comparison to DDR3's 6.4-12.8GB/s--20-40 times faster. Even the slowest DDR1 can manage 1600MB/s, 5 times faster than the theoretical max of a current SSD (which nothing currently can hit).
     

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