[CentOS] OT: What's wrong with RAID5
John R Pierce
pierce at hogranch.com
Thu Oct 1 05:20:36 UTC 2009
Stephen Harris wrote:
> "Almost always" is very dependent on the disks and size of the array.
> Let's take a 20TiByte array as an example.
he did say 'very large'.
note, raid10 has another parameter... say you have a 20 drive raid10 of
1TB drives (10TB total usable). if one drive fails, a rebuild only
requires reading one drive and writing the hotspare replacement, this is
fairly quick compared with the massive restripe operation of a raid5.
and, if during that rebuild operation, another drive fails, there's only
a 1 in 19 odds of it being the mirror of the previously failed drive if
we assume failures are a totally random occurance (yeah, ok, if we
assume that a drive is more likely to fail when its being accessed, then
the odds are soemwhat higher tha mirror would fail then another drive in
the array.... but, an array that does periodic sweeps on idle storage
will greatly reduce the possibility of this by 'discovering' a failing
drive much sooner.
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