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crash vs. power drop

crash vs. power drop

Posted Sep 14, 2009 14:13 UTC (Mon) by nye (guest, #51576)
In reply to: crash vs. power drop by ncm
Parent article: POSIX v. reality: A position on O_PONIES

This is why barriers *should* be a good thing.

If all you care about is benchmark results, there's an obvious incentive for the drive to claim falsely that some data has been physically written to disk.

On the other hand, there's far less incentive to lie about barriers. If all you're saying is that '*if and when* this data has been written, then that other data will have also been written', you can still happily claim that it's all done when it isn't, without breaking the barrier commitment.

When you have that commitment, it's possible to build a transaction system upon it which works even under the assumption that the drive will lie. You're not going to achieve the full benchmark speed, but it's going to be far better than turning off the cache.

Of course, whether drive manufacturers see it that way is another matter. Is there any data on whether drives actually honour write barriers? It would be interesting to see if there are indeed drives that aren't expensive enough to report accurately on when data has been written, but still honour the barrier request.


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crash vs. power drop

Posted Sep 14, 2009 15:06 UTC (Mon) by ncm (subscriber, #165) [Link]

This is where Scott Adams's "Which is more likely" principle is useful. We just frame the question, thus:

Drive manufacturer A can sell almost equally as many drives made this way as that way, but "that way" costs more development time and might make it come out a little slower in benchmarks. Some purchase decisions depend on claiming it's made "that way". The manufacturer can make it "that way" or just say it is, but not. Which is more likely?

crash vs. power drop

Posted Sep 14, 2009 16:12 UTC (Mon) by nye (guest, #51576) [Link]

A fair point, but in all things there's a strong economic incentive to claim that a product does something it doesn't, if that will make more people buy it, and yet most things don't claim do do something which is simply factually not true.

Usually when it's a feature that either works or doesn't - so it's not a subjective measurement - it's likely that a product does at least technically do what it says it does.

Presumably the argument is that the manufacturers aren't specifically claiming a particular feature, but the disk is behaving in a particular way that just happens to be not what the user expected - so they're not technically lying. This does seem to weaken the idea that they're doing it to improve the chances of people buying it though, if they're not stating it as a feature.

Just out of interest, I've just spent a while trying to see if I can find out what the difference is between the Samsung HE502IJ and HD502IJ - two drives which are identical on paper, but one is sold as 'RAID-class'. Neither are even remotely expensive enough not to lie about their actions, so what's the difference? Well, some forum post claims that one has a 'rotational vibration sensor', whatever that means.

In conclusion, people who try to sell you things are all liars and cheats, and I intend to grow a beard and live out the rest of my days as a hermit, never having to worry about these things again. Perhaps I shall raise yaks.

crash vs. power drop

Posted Sep 15, 2009 12:52 UTC (Tue) by Cato (subscriber, #7643) [Link]

I believe that TLER (time limited error recovery) is one characteristic of enterprise drives - simply means fewer retries so that the RAID controller or OS gets a drive failure more quickly, and knows to use redundancy to complete the I/O.

crash vs. power drop

Posted Sep 15, 2009 12:49 UTC (Tue) by Cato (subscriber, #7643) [Link]

Maybe the best way round this is to make sure that performance benchmarks always include a 'reliability benchmark' that detects drives which are lying about writes to hard disk.


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