On Jul 5, 2009, at 9:20 PM, Richard Elling <richard.ell...@gmail.com>
wrote:
Ross Walker wrote:
Thanks for the info. SSD is still very much a moving target.
I worry about SSD drives long term reliability. If I mirror two of
the same drives what do you think the probability of a double
failure will be in 3, 4, 5 years?
Assuming there are no common cause faults (eg firmware), you should
expect an MTBF of 2-4M hours. But I can't answer the question without
knowing more info. It seems to me that you are really asking for the
MTTDL, which is a representation of probability of data loss. I
describe
these algorithms here:
http://blogs.sun.com/relling/entry/a_story_of_two_mttdl
Since the vendors do not report UER rates, which makes sense for
flash devices, the MTTDL[1] model applies. You can do the math
yourself, once you figure out what your MTTR might be. For enterprise
systems, we usually default to 8 hour response, but for home users you
might plan on a few days, so you can take a vacation every once in a
while. For 48 hours MTTR:
2M hours MTBF -> MTTDL[1] = 4,756,469 years
4M hours MTBF -> MTTDL[1] = 19,025,875 years
Most folks find it more intuitive to look at probability per year in
the
form of a percent, so
2M hours MTBF -> Annual DL rate = 0.000021%
4M hours MTBF -> Annual DL rate = 0.000005%
If you want to more accurately model based on endurance, then you'll
need to know the expected write rate and the nature of the wear
leveling
mechanism. It can be done, but the probability is really, really
small.
Wow, detailed, interested in a career in actuarial analysis? :-)
Thanks, I'll try to wrap my mind around this during daylight hours
after my caffeine fix.
What I would really like to see is zpool's ability to fail-back to
an inline zil in the event an external one fails or is missing.
Then one can remove an slog from a pool and add a different one if
necessary or just remove it altogether.
It already does this, with caveats. What you might also want is
CR 6574286, removing a slog doesn't work.
http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6574286
Well I'll keep an eye on when the fix gets out and then for it to get
into Solaris 10.
-Ross
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