On Tue Nov 01, 2011 at 05:32:17 -0400, Miles Fidelman wrote:

Robin Hill wrote:

quoted

On Tue Nov 01, 2011 at 04:13:26 -0400, Miles Fidelman wrote:

quoted

David Brown wrote:

quoted

No, md RAID10 does /not/ offer more redundancy than RAID1.  You are
right that md RAID10 offers more than RAID1 (or traditional RAID0 over
RAID1 sets) - but it is a convenience and performance benefit, not a
redundancy benefit.  In particular, it lets you build RAID10 from any
number of disks, not just two.  And it lets you stripe over all disks,
improving performance for some loads (though not /all/ loads - if you
have lots of concurrent small reads, you may be faster using plain
RAID1).

wasn't suggesting that it does - just that it does things differently
than normal raid 1+0 - for example, by doing mirroring and striping as a
unitary operation, it works across odd number of drives - it also (I
think) allows for more than 2 copies of a block (not completely clear
how many copies of a block would be made if you specified a 16 drive
array) - sort of what I'm wondering here

By default it'll make 2 copies, regardless how many devices are in the
array. You can specify how many copies you want though, so -n3 will give
you a near configuration with 3 copies, -n4 for four copies, etc.

cool, so with 16 drives, and say -n6 or -n8, and a far configuration - 
that gives a pretty good level of resistance to multi-disk failures, as 
well as an entire node failure (taking out 4 drives)

Sorry, my mistake - it should be -p n3, or -p n4. You'll want -p f6/-p
f8 to get a far configuration though, but yes, that should give good
redundancy against a single node failure.

which then leaves the question of whether the md driver, itself, can be 
failed over from one node to another

I don't see why not. You'll probably need to force assembly though, as
it's likely the devices will be slightly out-of-synch after the node
failure.

Cheers,
    Robin
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