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EVA 4400 single or double spare...

Posted on 2010-08-21
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When is it a bad idea to do double spare?  Does it hurt IOPS?  Does it help?

Currently I have 18 450's.  Going to add 6 more in a week.

Moved off of 146g disks (18) killed that disk group and retired disks.

I have 3 disk shelf's.

So I just wanted to get the skinny on double protection and if it was dumb.

We selected double becuase we just don't want the array to fail if we can help it.
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Question by:smcpartlin
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by:SelfGovern
ID: 33494913
There is a slight performance difference for writes between RAID 5 and RAID 6; for reads there won't be anything noticeable.
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by:smcpartlin
ID: 33497212
On the Eva 4400 double protection is separate from the raid level.  

On our Eva 4400 raid 6 is not even selecte-able.
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andyalder earned 500 total points
ID: 33500726
Single reserves 2 disks worth of data as distributed hot spares, double reserves 4 disks worth of data. It has no performance difference since the spares are distributed across all the disks a bit like a small RAID5 stripe. The spares aren't even created until needed, it's just reserved space so if you have enough room currently and set it to double you can change it to single or even none if you need that space in future. Same with the disk space reserve setting, it just stops you creating more LUNs and sets off alarms when you hit the reserve, you can always change it when you feel like it without affecting anything else.

N.B. it's not quite two or four whole disks since it doesn't have to reserve a whole disk as it takes into account the fact that it doesn't have to spare up for reserve area.
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by:smcpartlin
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by:andyalder
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Why have you posted a dot?
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by:SelfGovern
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Perhaps because, as I hear, "Dot's nice".
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by:smcpartlin
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Thanks.
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