freebsd-fs Digest, Vol 472, Issue 5

[Radio młodych bandytów]

On 2012-07-06 14:00, freebsd-fs-request at freebsd.org wrote:> It's easy to 
find the failure math for raidz2 and raidz3.
 >
 > But what if you create a pool with 3 different raidz3 vdevs inside of 
it ?
 >
 > For instance, 3 12-drive raidz3 vdevs in one big pool.
 >
 > For each individual vdev the failure probability is now higher, since 
not only will it fail when 4 drives in the vdev fail, but it will also 
fail if four drives in any of the other two vdevs fail.
 >
 > So each raidz3 vdev now has a failure rate higher than vanilla raidz3 
... but what is that new failure rate ?  Is it still higher than vanilla 
raidz2 ?

Skip these calculations. They all assume that drive failures are 
independent and it's not the case in the real world. There's been a good 
study on the topic of drive failures several years ago, 
http://static.usenix.org/events/fast07/tech/schroeder/schroeder_html/index.html
Among other findings they say:
"We also present strong evidence for the existence of correlations 
between disk replacement interarrivals."
-- 
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