reliability of SSD vs. hard disk

Discussion in 'Mac Basics and Help' started by motulist, Jun 30, 2010.

  1. motulist macrumors 601

    motulist

    Joined:
    Dec 2, 2003
    #1
    Googling around, I've seen very conflicting stories from different sources. Some sources say that modern SSDs are much more reliable than hard disks, and I've heard other sources say the opposite. If you've got a decent understanding of this topic, I'd like to hear what you have to say. I'm looking for the absolutely most reliable storage that I can buy for a reasonable price. I'm looking to buy 2 backup drives.

    Drive #1 I'll be using to backup my computer every week (through TimeMachine).

    Drive #2 I'll write to at most once a year, and I need this drive to be readable if I go to access it after many many years of it sitting in a cool dark dry place.

    Reliability is paramount for this project. Do you have an good info on this topic?
     
  2. Makosuke macrumors 603

    Joined:
    Aug 15, 2001
    Location:
    The Cool Part of CA, USA
    #2
    Unless we're talking about an exceedingly small amount of data here, given the price-per-gigabyte difference between SSDs and rotating magnetic media, SSDs make no sense whatsoever for backup.

    Look at it this way: You're going to pay ~$200 for 50-100GB of SSD (depending on the quality of the drive), and ~$70-100 for a 1TB-1.5TB HD.

    If you have 50GB of data to back up, you could buy two magnetic hard drives for the price of one SSD, back up on both, and if one dies you have a second working one.

    If you have 500GB of data to back up, even a single-layer SSD backup is going to cost you on the order of $1000. For that price you could comfortably buy TEN rotating hard drives, put the same data on all ten, buy a fire-safe with the extra you saved, and unless ALL TEN drives fail simultaneously (because somebody EMP'd your house, in which case you probably won't much care about your backup strategy, because you either just got nuked or western civilization was devastated by a geomagnetic storm), you're fine. And of course you'd really want TWO SSDs, since if you only have a single backup things could go horribly wrong if it suffers an infant failure when you need it, or your computer fries it as soon as you hook it up to do a restore, so double the cost.

    Or, buy two 1.5TB HDs ($200), a BDR drive ($200), 500GB worth of BD-R or -RE media ($200), make two copies of your data, burn a copy to BDR, put one hard drive in your house, one in a safe deposit box, mail the BDRs to a family member to hedge against EMP attack, upload a full copy of the data to an online backup service, and you STILL probably only spent $800. On a solution that's going to be exponentially more reliable when it comes to data retrievability than even the most reliable SSD on earth.

    Me personally, I have one online backup at home and a second HD backup I keep at my parents' house, in case my place gets looted or burns to the ground, plus all the REALLY irreplaceable stuff (writing, mostly) is also stored on a web server with DreamHost. This cost me about $200 for over 1TB worth of backup, plus around $100/year for the few dozen MB of online stuff that I'd be paying anyway for webhosting. I'd put that up against any SSD at a minute fraction of the cost.

    Thing is, when people are talking about SSD reliability, they're talking about how likely the drive is to fail while it's being used. Which is entirely different from reliability of something sitting quietly in a box for backup purposes. When people say SSDs are unreliable, they're most likely talking about the fact that an individual cell of an SSD can only be re-written a finite number of times, after which it becomes useless. In some use patterns, with a lot of data being written and re-written (say, a Photoshop scratch disk), this limit might actually come into play, and cause the disk to fail sooner. On the other hand, an SSD is nearly immune to damage due to getting dropped or banged around, so in a laptop it's probably more reliable, at least physically. Similarly, traditional hard drives have moving parts, which do eventually wear out if they're left on long enough and/or turned on and off enough. SSDs are, presumably, significantly less prone to this sort of wear-related failure, on account of having no moving parts.

    Further, most SSDs have at least a little overprovisioning--extra, unused storage space to transparently swap in when a cell becomes unusable due to whatever failure occurred, either being overwritten one too many times or a simple component failure. Rotating hard drives have extra sectors, too, but a lot less of them, since the failure rate is lower. The more overprovisioned space in an SSD, the more reliable--at least on paper--it is. The best drives, like the enterprise-grade SandForce-based ones (for example, the top-of-the-line drives from OWC) have something like 30% extra, unused space to swap in--that's a lot (which you of course pay for).

    Based on pure MTTF statistics, most "normal" rotating hard drives are in the 1 million hour range (meaning, statistically and based on accelerated testing, that a drive will last an average of 1 million hours). This of course has little to do with reality--1 million hours is 114 years, and it's pretty obvious the chances of a drive lasting that long are just about zip, let alone an average drive lasting that long. The numbers are really only useful for comparisons. OWC's SSDs currently list MTTFs of 2 million hours, which is comparable to a good pro-grade rotating hard drive. (Interestingly, they originally listed 10 million hours on their top-of-the-line RE edition SSDs, but the site now seems to say 2 million on everything.)

    All that is to say that, according to manufacturer specs, a very good SSD is somewhere between 2x and 10x as reliable as a decent HD. On paper. In practice, none of these SSDs have been in the wild long enough to have any idea how reliable they'll actually be over time. Probably good, but who knows.

    None of that matters, though--the point is, when it comes to the ability to retrieve data, the more copies of your backups you have, the better your chances, and adding copies in other locations or other kinds of media (optical, for example) is that much better. And given the extravagant price-per-GB costs of an SSD, even if they were exponentially more reliable--which is debatable--they'd be a ridiculous choice for backup unless you're filthy rich. And even then, you'd probably do better to have an extra copy on rotating magnetic media instead of two SSDs, on the chance that the SSDs both suffer from the same design or firmware failure after X number of hours.
     
  3. miles01110 macrumors Core

    miles01110

    Joined:
    Jul 24, 2006
    Location:
    The Ivory Tower (I'm not coming down)
    #3
    You want the best technology for the cheapest price, which you can't have in this case. You get what you pay for- either spend X dollars on one small SSD drive or X dollars on multiple traditional drives.
     
  4. wjlafrance macrumors 6502

    Joined:
    Dec 23, 2009
    Location:
    Madison, WI
    #4
    If I understand correctly, BDR's aren't vulnerable to EMP since they're not electronic devices.
     
  5. Makosuke macrumors 603

    Joined:
    Aug 15, 2001
    Location:
    The Cool Part of CA, USA
    #5
    Probably a little more accurate to say "less vulnerable" since if you get a high enough EMP the induced currents will probably burn just about anything metal (I could be misunderstanding the effect, but I believe something like this is what fries CDs in the microwave). Of course, a rotating hard drive, being encased in a metal shield, is also a LOT more resistant to magnetic fields than people think, particularly if it's connected to something and therefore grounded.

    Unless you're in government continuity or get regularly struck by lightning, EMP would be way, way down on the list of things I'd be planning for in terms of backup strategy. I was just mentioning it as an example for the level of paranoia you could afford in place of trying to use SSD as a backup medium.
     
  6. wjlafrance macrumors 6502

    Joined:
    Dec 23, 2009
    Location:
    Madison, WI
    #6
    In reality, if I had to suggest the most affordable and usable backup solution, it would be an AirPort Express + 1TB external hard drive. I've dropped my external hard drive several times and it's never lost data, so I think one sitting on a shelf will do reasonably well. If you're paranoid about it, you can get a second one and make a weekly block-by-block copy of it with Disk Utility. Total cost would probably be in the range of $150 or $300 for the drives, plus $100 for the AirPort Express.

    EDIT -
    Or you could totally bypass the AirPort Express and just plug the hard drive into your computer.
     

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