Raid 1+0

[Valeri Galtsev]

On Tue, April 19, 2016 9:35 am, Steve O'Hara-Smith wrote:
> On Mon, 18 Apr 2016 17:02:57 -0400
> "Kevin P. Neal" <kpn at neutralgood.org> wrote:
>
>> On Mon, Apr 18, 2016 at 09:07:07PM +0100, Shamim Shahriar wrote:
>> > On 18/04/2016 20:22, Bernt Hansson wrote:
>> > > Hello list
>> > >
>> > >
>> > > Used gstripe to stripe the arrays raid/r0 + r1 into stripe0
>> > >
>> > Hi
>> >
>> > I'm sure there are people with more expertise than I am, and they can
>> > confirm either ways. But in my mind, given that you used RAID1 first
>> > (mirror) and then used those two RAID1 to create a RAID0, this is
>> > logically RAID 1+0. In other words, if you lose one disc from each of
>> > the RAID1 you are still safe. If you lose both from one single mirror
>> > array (highly unlikely), the stripe is unlikely to be of any use.
>>
>> Not that unlikely. If you take identical disks from the same company and
>> subject them to identical load then the probability that they will fail
>> around the same time is much higher than random.
>
> 	The classic case is not so much same model and same company but
> same batch which (at least in the early days of RAID) was quite likely.
> The
> more similar the drives and their long term load the more likely they are
> to fail at around the same time especially under the added load of
> replacing the first to fail.

Never happens to me that way. So my own small statistics of only a couple
of dozens or RAIDs used over decade and a half time is kind ofin agreement
with probability theory which IMHO says "double failure event" is much
less likely than single failure event. (See my longer post about what I do
to avoid apparent "double failure event", which are not actually such).

Valeri


> Perturbing that similarity is the goal.
>
>> That's why when I set up a mirror I always build it with drives from
>> different companies. And I make it a three way mirror if I can.
>
> 	I like to use a variety of drives and ages if I can, and yes two
> drives of redundancy either three way mirror or some variant of RAID6 (I
> like RAIDZ2).
>
> --
> Steve O'Hara-Smith <steve at sohara.org>
>
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++++++++++++++++++++++++++++++++++++++++
Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247
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