On Feb 16, 2010, at 4:47 PM, Christo Kutrovsky wrote:
> Just finished reading the following excellent post:
>
> http://queue.acm.org/detail.cfm?id=1670144
>
> And started thinking what would be the best long term setup for a home
> server, given limited number of disk slots (say 10).
>
> I considered something like simply do a 2way mirror. What are the chances for
> a very specific drive to fail in 2 way mirror? What if I do not want to take
> that chance?
The probability of a device to fail in a time interval (T) given its MTBF (or
AFR, but be careful about
how the vendors publish such specs [*]) is:
1-e^(-T/MTBF)
so if you have a consumer-grade disk with 700,000 hours rated MTBF, then over a
time period of 1 year (8760 hours) you get:
Pfailure = 1 - e^(-8760/700000) = 1.24%
> I could always put "copies=2" (or more) to my important datasets and take
> some risk and tolerate such a failure.
+1
> But chances are, everything that is not copies=2 will have some data on those
> devices, and will be lost.
>
> So I was thinking, how can I limit the damage, how to inject some kind of
> "damage control".
The problem is that MTBF measurements are only one part of the picture.
Murphy's Law says something will go wrong, so also plan on backups.
> One of the ideas that sparkled is have a "max devices" property for each data
> set, and limit how many mirrored devices a given data set can be spread on. I
> mean if you don't need the performance, you can limit (minimize) the device,
> should your capacity allow this.
>
> Imagine this scenario:
> You lost 2 disks, and unfortunately you lost the 2 sides of a mirror.
Doing some simple math, and using the simple MTTDL[1] model, you can
figure the probability of that happening in one year for a pair of 700k hours
disks and a 24 hour MTTR as:
Pfailure = 0.000086% (trust me, I've got a spreadsheet :-)
> You have 2 choices to pick from:
> - loose entirely Mary, Gary's and Kelly's "documents"
> or
> - loose a small piece of Everyone's "documents".
>
> This could be implement via something similar to:
> read/write property "target device spread"
> read only property of "achieved device spread" as this will be size dependant.
>
> Opinions?
I use mirrors. For the important stuff, like my wife's photos and articles, I
set
copies=2. And I take regular backups via snapshots to multiple disks, some
of which are offsite. With an appliance, like NexentaStor, it is trivial to
setup
a replication scheme between multiple ZFS sites.
> Remember. The goal is damage control. I know 2x raidz2 offers better
> protection for more capacity (altought less performance, but that's no the
> point).
Notes:
* http://blogs.sun.com/relling/entry/awesome_disk_afr_or_is
** http://blogs.sun.com/relling/entry/a_story_of_two_mttdl
-- richard
ZFS storage and performance consulting at http://www.RichardElling.com
ZFS training on deduplication, NexentaStor, and NAS performance
http://nexenta-atlanta.eventbrite.com (March 15-17, 2010)
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