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DragonFly users List (threaded) for 2012-08
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Re: solid-state drives

From: Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>
Date: Fri, 3 Aug 2012 10:17:53 -0700 (PDT)

    Well, dedup has fairly low overhead so that would be fine on a SSD
    too, but because SSD's tend to be smaller than HDDs there also tends to
    be not so much data to dedup so you might not get much out of enabling


    The SSD's biggest benefit is as a cache, though I don't discount the
    wonderfully fast boots I get with SSD-based roots on my laptops.
    Random access read I/O on a SSD is several orders of magnitude faster
    than on a HDD (e.g.  20,000+ iops vs 250-400 iops)... that's a 50x
    factor and a 15K rpm HDD won't help much.

    Random write I/O is a bit more problematic and depends on many
    factors, mainly related to how well the SSD is able to write-combine
    the I/O requests and the size of the blocks being written.  I haven't
    run any tests in this regard, but something like the OCZ's with their
    massive ram caches (and higher power requirements) will likely do better
    with random writes than, e.g. the Intel SSDs which have very little ram.

    Linear read and write I/O between a SSD and a HDD are closer.  The SSD
    will be 2x-4x faster on the linear read I/O (instead of 50x faster),
    and maybe 1.5-2x faster for linear write I/O.

    NOTE!  This is for a reasonably-sized SSD, 200GB or larger.  SSD
    performance is DIRECTLY related to the actual number of flash chips in
    the SSD, so there is a huge difference in the performance of, say,
    a 200GB SSD verses the performance of a 40GB SSD.

    A 40GB SSD can be limited to e.g. 40 MBytes/sec writing.  A 200GB SSD
    with a 6GBit/sec SATA phy can do 400 MBytes/sec writing and exceed
    500 MBytes/sec reading.  Big difference.

					Matthew Dillon 

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